G4 400 PCI won't boot... Help, Please.

Hi...

I have an old G4 400 PCI that has worked without a problem for ~8 years. It hasn't been modified since it was first set-up, except I added a 2nd internal 120 GB hard drive about 5 years ago. The original hard drive died about 6 months ago, so I disconnected it and installed OS X 10.3 on the 120 GB drive and it has worked fine ever since. The computer is primarily used by my 9 y.o. daughter, but I leave it on all the time because I keep some old files on the hard drive which I access via my home network.

My problem started when I turned the computer off before leaving on a 10 day trip. After returning, it would no longer start up. I makes the start up sound, then shows a grey Apple on a white background for 10 or 20 seconds, then goes into a kernel panic ("backtrace terminated - unaligned frame address" blah, blah, blah).

My first thought is that maybe it was the battery (it still had the original one). So I bought a new one and installed it, but no change.

Other things I tried:
• Disconnect everything except the monitor, hit the reset button (the one next to the battery), and restart. No change.
• Take out the RAM (2 ea. 128 MB and 2 ea. 256 MB), and reinstall it one at a time and restart. No change. Reinstall it in pairs and restart. No change.
• I tried to force start up from a CD, but the start up process never gets far enough to recognize the keyboard. So I disconnected the hard disc. With no CD in the CD drive, the computer just displays a flashing ? alternated with a flashing folder with a mac face icon. If I put a CD in the CD drive, the computer seems to try to load it, shows the grey Apple icon, the does the same kernel panic thing as when the hard disk is connected.

So that's about as far as my limited knowledge will take me. Any suggestions would be greatly appreciated. My daughter is going through Webkinz withdrawal.