Symptoms: the computer freezes up completely, becoming unresponsive to keystrokes, mouse movement, and even the reset key. Whatever I was doing at the time remains on the screen, it doesn't go black; the hard drive LED goes on and remains on. I suspect that the lockups are somehow video-related, because they almost always happen when the system has been doing 3D video stuff (Operation: Flashpoint, FS2002, Deus Ex, etc, even the screensaver occasionally).
If I power down the system using the manual switch on the back and leave it for a while (at least a half hour), it operates normally again when I turn it back on. Turning it back on immediately results in a black screen and an illuminated HD LED, but no other activity; once, it got as far as the "Press <DEL> to enter setup" screen and then froze completely in the same manner.
This problem has occurred using the integrated video, using an old GeForce2 card and using my current Radeon 9000 card.
System specs: AMD Athlon 1.0 GHZ; 640 MB PC133 RAM (128 + 512); video card = Radeon 9000 Pro (64 MB); running WinXP (SP2); onboard audio.
Thanks in advance for your help.