MAJOR system problems

I'm not sure what to do anymore....
I was running OS X.2.8 happily for a couple of years on a G4 dual 867. About 3 months ago I started getting semi-frequent crashes...maybe about once a week or two. Then about a week ago I was listening to an internet radio broadcast in iTunes, and uploading about 500 MB of jpegs to my site and my computer died right in the middle of it. I rebooted, and that's when the following symptoms started occuring:
--taking up to 5-8 minutes to boot
--desktop boots but I can't click on anything or open a program (mouse moves around fine though)
--computer boots up only to grey apple screen
--computer boots up and hangs on 'Checking local disks' for up to 7 minutes
--crashes intermittently whenever I open a program, move a window between the 2 monitors (both running off a dual-head card), or sometimes just crashes whenever it's just sitting there for more than 30 minutes.
--crashes in different ways including kernal panics, spinning beach ball of death, or just unresponsiveness to any clicking.

fun.

soooo, I do have a 120 gb backup drive, so I installed the basic OS X.2 onto that, & was able to backup all my files onto that (in between crashes). I ran apple hardware test disk and it said everything was normal. I was getting ready to wipe the main drive and reinstall everything, when I booted it up and it won't even see the main drive anymore. So i'm thinking, OK--just a bad main drive.
But the symptoms I listed earlier are happening while booted off my backup drive with the absolute basic setup--haven't installed any software, drivers, etc.
This feels like a virus, but I've searched and haven't found anyone complaining about similar problems with a virus. I've tried running unconnected to the internet and still have the same problems.
One thing I haven't tried that I'm probably going to is to physically take the main drive out (which isn't even recognized by the cpu anymore)....

Does anyone have ANY suggestions? I'm tearing my hair out here.
Thanks so much,
aaron