First of all, Power Mac G5s are towers and were quite easy to mess with. Sounds like you are talking about an iMac. My friend has one of those, and he told me that to replace parts inside, you just pop 2 screws on the base out to remove the back. From there, the whole machine is accessible and is easy to work on. He said Apple even sent his friend a new hard drive and repair parts to fix a dead drive a couple of years ago. No voided warranty or anything. Not bad.
If everything you say is true, then perhaps the logic board is fried from something. Those G5 chips ran hot. My G4 runs fine over here, and it is quite a bit older than whatever you are probably working with. I'm thinking that the owner of this machine must have done something to provoke this problem. Otherwise, something failed and it may be impossible to repair. That computer might as well be replaced at this point in time. The Intel Macs are very reliable and well-built.
Run programs at startup? Many apps I have will load upon powering on. Under the Accounts pane in System Preferences, you have a list of Startup Items. You can get should be able to run anything after logging in. I don't see what you are getting at.
You can reinstall the system with a regular Leopard disk. You can get a copy at Apple or Amazon, etc. The restore disks are shipped along with the Mac to allow the user to "restore" the computer to the system it came with and to give you some utilities and bundled programs. You should be able to run hardware tests off of the restore disks. If the owner has more recent software, you could try that route and install newer copies of iLife or whatever he or she is using on it.
There should be a little hole or something on the slot-load drive which will automatically eject the disc if it should get stuck, from what I've been told. I've also heard some people say managing Macs on their networks has been pretty good for them. This iMac may not run most builds of Linux considering that it uses a PowerPC chip and there are more Linux systems available for x86 architectures. The new Intel Macs can run Windows or any x86 OS natively.
Newer Macs simply work better, and I recommend you try using one and comparing the two. Condemning all of Apple's products for the malfunction of one of their older machines isn't very reasonable. Good luck working with that G5.
Background - I'm an IT manager responsible for over 400 computers spread accross 5 states all of which are PC's.
So my boss comes in with his wifes G5 last week saying he thinks shes got a virus and wanted me to fix it. I start the thing up and it gives me the Mac screen of death - the "You need to reboot your computer" message in 20 different languages. So I reboot and get the same message over and over. I read somewhere online that maybe it's a memory problem. So I replace the memory. Same problem - didn't fix it.
After some fiddling, I am able to get the damn thing to actually boot and go to repair the permissions on the hard disk. It fixes them no problem and I reboot only to get the same damn Mac screen of death.
My boss brings the OS X discs that came with the computer so I can use the hardware test. The extended test runs and everything comes back with no problems or errors but I'm still getting the screen of death.
Next thing I thought was maybe the hard drive is going bad. Without any other way to test it, I decided just to buy a new one. That was cheaper than buying the software to test it with.
I get the new hard drive today, take out 20,000 screws to install it and finally get it up and running. Boot up to the OS X installation disk, partition the hard drive and start the installation. It asks for the 2nd disk, I put it in and it continues the installation.
Then about 50% into it, it gives another damn error message saying the installation failed. After searching for this new problem, I come to find out some of the OS X disks that went out with the G5's had a corrupt 2nd installation disk. The solution was to take any upgrades out (new memory, hard drive) and try again. So I took the memory out (left the hard drive in for obvious reasons) and started over.
Put disk 2 in and get the same crap - installation failed. So I went to just start over with disk one but it goes to a bunch of text instead of booting to the disk saying it failed to load a driver of some sort.
I'm at my wits end, about to throw this POS out the window (if I had one in my office hehe)...
I just don't understand for the life of me why someone would want to buy a computer that they have no control over. You can't create a startup disk to run your own programs at startup, it won't boot from linux based disks like normal computers do. You have to have your specific OS X installation disk for it to reinstall the OS. It takes years of rebooting and knowing what stupid key to hold down just to get the damn disk to eject. Is it soooo hard just to put a button on the disk drive to eject it or is that not cool enough for Mac users? Is it sooo hard to allow someone to make a boot disk to run their own utilities without making me go pay $100+ for whatever Mac only software is available to test my hardware?!?!? And they say Microsoft nickle and dimes you...
I'm just ranting...this is only the 2nd Mac I've worked on and to be honest I don't see the draw to these things. From an IT standpoint - these things are a freaking nightmare. I hope I never see another Mac again...