I run Windows XP SP2 on a Desktop I built (I know that doesn't provide much, but I can't say "on a Dell X103938R" or something).
It has 'seized' about 7 times in its year and a half life. Everything freezes. I can't move the mouse cursor and the keyboard seems unresponsive. I can turn the monitor on and off and it will hold the last image. The light for the mouse is responsive if I move it. The keyboard lights change when pressed (cap locks, etc).
I've waited upto ten minutes for a change, nothing.
I haven't connected any activity to the seizing. It's happened when all I was "running" were fullscreen programs (games), just checking email, or once when I was sitting at my desktop (I was reading a book and when I tried to use the mouse, nothing).
I've never been able to figure it out. I have to hard reset, and then its fine. It doesn't file system check or anything (not sure if Windows does that). No error when I load up the computer, nothing.
If I had uTorrent open, it will have to recheck the torrent files to make sure they weren't corrupted though. (It's not always open when it seizes either).
I'm using an AMD Athlon 5400+ with a NVidia GeForce 8600 GT, if that helps. I'm using two hard drives, 500Gb Wester Digital with a 1Tb Hitachi.