John,
For what it's worth, MOST stores are going to be honest for the most part about this sort of thing. There may, however, sometimes be issues when you take a box in for service and MIGHT not get quite the box you took in.
Case in point: You have an older computer - one that's say 3 - 4 years old and some component is failing - say, a video card. Your computer's motherboard has an 8x AGP based bus. At the time you built this computer, you paid good money for a particular video card but now, 3+ years later, that particular vendor is no longer in business or the specific model of card in question is obsolete and an exact replacement can't be found. Now at this point, if the computer repair place is honest, they will probably call you on the phone and get further instructions as to what you want them to do about it. More often than not, they'll recommend a replacement card they have in stock or can get ahold of easily. Chances are it may not, however, be an exact match for the one being replaced. It may be better than the one in the box or then again, it may not.
Before taking the computer anywhere - the FIRST thing to do is BACK UP YOUR DATA! Of course, if your computer won't boot any more, then this bit of advice is a day late and a dollar short.
So how does one NOT get ripped off?
1.) The first too in your arsenal is to KNOW YOUR HARDWARE!
Unfortunately, for the vast majority of people (read: those who aren't enthusiasts) probably wouldn't know a CPU chip from a RAM chip. And quite frankly could care less what the exact particulars are as long as it does what they want and expect it to do. Unless they're into gaming, they might not even notice a degrade in video performance. If this describes you, your best bet is to get some help from the geek in the family or the one down the street.
It's best to know what you've got on the before side of the picture. A trip through the Device Manager (right-click on My Computer, select Manage and the Device Manger item on the left hand tree of the computer managment console.). Go through each item on the tree and see what you've got for each component. Write this info down somewhere. If you've got an AMD Athlon XP 2100+ processor, make a note of it. If you've got 512 MB RAM, write that down too. Keep going through the main sections. Don't worry too much about external devices - such as printers, external hard drives, other things plugged in via USB.
2.) When you go into a place, get a feel for the guys working behind the counter. If you're taking it to a small shop, look around and ask questions. If the guy seems to know what he's talking about, and MOST importantly, you feel comfortable with dealing with the guy, by all means, proceed with getting your computer repaired. However, if you've got a nagging feeling in the back of your head, or you feel the slightest doubt that the guy's honest OR if he's jumping the gun and telling you that you really need to dump everything and Upgrade! UPGRADE! UPGRADE! then you just might want to go elsewhere. This guy's got only ONE thing on his mind - and it isn't your best interest.
If you're taking the computer to a big named store, odds are the guy may NOT know a CPU from a hole in the ground. Big name stores tend to hire people who will work for less money. More often than not, they will hire younger people who at best, know enough about computers to be slightly dangerous - in other words, they can install basic hardware upgrades, but they're not quite able to diagnose or fix anything serious. When it comes to handling serious problems, they will take custody of your computer, give you a claim check/repair order, tell you it will take some time to get the machine looked at and fixed and send you home. If you happen to have a warranty (extended or otherwise), you may (or may not) also be given the option of getting a loaner computer for the duration. They will generally then send your computer to a larger facility that does repairs for all of the stores in a given area.
The downside to the big named store is that they generally tend to take a really, REALLY long time getting things fixed and back to you. And even then, there still might be issues that never quite get resolved.
Case in point: A colleague of mine has a client who bought a computer from Fry's Electronics complete with a 5 year extended warranty. About 4 years and 6 months into the warranty, the computer was no longer able to access the guy's network. So, they took it into Fry's for service - FIVE times. Each time, they would take the machine in, get the repair order, get a loaner, and wait SIX weeks only to get the computer back, allegedly fixed and yet, it STILL didn't work properly. A call to the tech support area at Compaq determined the problem was most likely a motherboard issue and it would have to be replaced. Each time, however, the techs at Fry's simply replaced the network card and sent it back. On the fifth and final time they took the thing in, Fry's actually lost the entire machine. They wound up giving him a brand new one.
Had the guy NOT bought the extended warranty, my colleague could have diagnosed the problem, replaced the motherboard with a new one and the guy could have been back up and running in a day or two at the most. As it was, it took 5 trips to the store, 6 weeks between dropping it off and getting it back - for a total of 30 odd weeks. That's more than 6 months. Ultimately, yes, he got some satisfaction out of it BUT he had to put in some serious effort on his part to get it.
3.) ASK AROUND! Ask your friends who have computers where they take their computer issues. Chances are, if they've had problems in the past, they can steer you to good, reliable vendors and away from those who aren't.
4.) Dirty Trick: Use the old "drop of nail polish on the components" bit. If your options are limited and the only game in town is someone you aren't 100% comfortable with, here's a trick that's been around the block a few times. Buy yourself a small bottle of nail polish. The color doesn't really matter as long as it's not clear. (You want the mark to be visible.) Simply open up your case and put a very small drop of the polish on each component - making sure you're NOT putting it on any of the metal legs where parts are soldered onto the motherboard. You do NOT want to compound your problems by shorting something out. A small drop on the edge of your motherboard, sticks of RAM, CPU fan, hard drives, any cards plugged into the computer, etc... will suffice. It doesn't have to be obviously placed as long as you can remember where you put them.
Close the case and take it in for service. When you get it back, you can pop the case open again and look for the drops of nail polish. If something was swapped out that shouldn't have been, you can nail the guy for making unauthorized changes.
There are occasions when this trick won't work. If for whatever reason, the techs decide that your computer can't be repaired and is still under warranty, they may opt to give you a refurbished model that is at the very least, equal to or (preferrably) better than what you walked in with.
Some companies routinely skip trying to figure out what's wrong with the computer and directly issue a replacement. In a situation like this, you will most certainly need to take a few extra steps to make sure you don't lose anything important. (You DID back everything up - Right?)
Case in point: Sony VAIO laptops. About a year or so ago, one of my clients had a problem with his laptop. One of the keys on the keyboard went missing. To get it fixed, Sony's techs told me they would simply swap the laptop out. I could NOT buy a replacement keyboard from them at ANY price. They informed me that any upgrade, namely additional RAM, would NOT be included in the replacement laptop. Furthermore, the data on the hard drive would likewise be lost. Not to mention, if you changed the partitions on the laptop, they would be MOST unhappy. If you upgrade the OS (the one in question came with Windows ME and was upgraded to XP.), you were likewise outta luck - the replacement will the orignal OS - Windows ME on it.
In other words, they want the laptop shipped back to them EXACTLY as they sold it. Upgrades and embellishments are heavily frowned upon. Makes you wonder exactly WHO owns the laptop - you or Sony...?
5.) IF you can, get ahold of any and all serial numbers associated with the hardware inside your computer. Sometimes, however, this would require taking the computer apart. Once again, if you're not comfortable doing this, get someone to help you out. This would include, most importantly, the product key for your copy of Windows from the sticker on the side/back/top/bottom of the computer. As with the nail polish trick, make a list ahead of time and compare it with the computer you get back. Note any discrepancies and demand an explanation.
6.) Above all... Be sure to request ALL of your old parts back. Just as if you were taking your car in for service, you have the rights to get all of your old components back. Make sure you explain this to them when you take the computer in and BEFORE you sign the work order. Better still, make sure that information is written legibly on the work order so the techs know to save the parts and get them back to you. If you don't tell them to do so, odds are they will dispose of the evidence. If they balk at the idea of getting the replaced parts back to you, you might consider going elsewhere.
7.) A Picture is worth a 1000 words... Take pictures of the insides of your computer. Make sure you've got plenty of light, use a flash if needed. Make sure the pictures are crystal clear. If you notice anything odd - like your formerly green motherboard is now royal purple, or the big old honkin' fan on your video card is different, you know what to do - especially when you took the computer in for something else.