I used to do it because I once needed, and gratefully received, help from those who had better skills and more experience than me. Then I did it because I sometimes have better skills and more experience [as I should after being a pro. for decades] than someone just starting out. I was repaying the community for the help I was given when I needed it.
And then there's persistance-uk who basically didn't bother to read what I wrote, said much of what I had already said but in poorer English and with little or no punctuation - and a poor grasp of capitalisation - and contaminated his message with dangerous and half-baked advice.
The "sfc" command does not work miracles. If a file is badly corrupted it may well be unfixable by anything short of magic. Worse, "sfc" may well replace a corrupt copy of a file with the "back-up copy" which is also corrupt, in truth that may be where the corruption came from. This is not progress. It's a useful tool but it isn't a panacea.Sometimes you just have to go to your original system restore disks and pull a clean copy of corrupted files from there.
On that subject, even if a run of sfc tells you files are corrupted, of if chkdisk does if you use that, if your box is running fine you need not fix them. It's *YOUR* computer and if you can live with a few broken files feel free to do so. If you haven't noticed them, the odds are they aren't being used much anyway. [And I do know how dangerous that advice is if you're an expert, but most of us just want our machines to *work*. If they work well enough with a few broken files, who cares?]
That last bit of "don't fix what aint broked" applies very much so to the Registry.
The Windows Registry is often huge and full of crud left over from programs you've deleted. Registry cleaners are supposed to remove all keys referring to those gone programs. Sometimes they do. Sometimes they also remove vital keys that tell Windows how to work. This can stop Windows working and convert your lovely machine into a brick. The best advice for working with the Windows Registry is simple: DON'T.
Get a professional to do it for you. Preferably someone Microsoft certified and trusted by his friends. Don't take your box into PCWorld or an Apple store and expect their "Geek Squads" to do the job. These people are sometimes so poorly trained a dyslexic pigeon would be safer hands. That's not an insult, just a fact. I will happily futz about with the Registry of my own boxes, and I've yet to break one, I've yet to even need to restore the Registry from a back-up, but before I mess with someone else's box I always tell them there is a very small chance I'll brick it and they'll need to pay a real Microsoft Guy to come fix it. I never have, but it's possible. If someone like me, with eyes and a brain can mess up the registry, think what an set of rules written for a generic Windows machine can do to your very personal copy. Remember, the program assumes your Registry is like every other. It isn't. No two are identical ten minutes after the machines are switched on. A human with skill can cope with these differences, a program often can not.
Registry cleaners can brick your machine.
Before even thinking of using one, make sure Registry.bak exists and make a backup of the real, operational one. Then do not use the software.
Disk defragmenters were handy for Windows up to wWin95, or even WinME. There is no need for such a thing in WindowsXP, Win7, W8 or W8-one.
http://support.microsoft.com/kb/314848 tells you how to defragment a WinXP disk. You do not need third party, add-on software. Ever. It's not quite as dangerous as a Registry Cleaner, mostly, but a defragger can brick your disk. They don't do it often but it's possible.
Before anyone else says it, the Microsoft Defragmenter that comes with WinXP can also kill the disk but that is also rare. So rare that I've yet to hear of it. It has probably happened [thousands of millions of disks and ten years or more of WinXP, *everything* has happened by now] but it's rare and the one advantage of the WinXP defragger is that it does come with WinXP. It was written *for* WinXP, it doesn't come with unnecessary baggage. It mostly just works.
My last advice to Marie M. about her XP re-install enquiry is : don't. There is no need. Just clean your machine. Do it slowly. Take online advice from the BleepingComputer and other experts. Type "online support" into Google for more local help. Do the job slowly and carefully and make sure you can reverse any damage you do by having back-ups. Learn as you go and it can even be fun.
Microsoft is stopping support for Windows XP but the operating system itself will still run as long as you machine does. Just be careful when or if you connect it to the Internet and you'll be fine.
So there you go, my last posting, complete with adjectives, good syntax, nice grammar and loads and loads of well-placed punctuation marks. It even has capitalised lettering.
Why do I bother? From now, I don't.