I have a similar problem... While playing games and sometimes other times my system rebooted itself... I turned off the auto restart in the properties of the system. But now I get physical memory dumps. (I know its not my RAM for many reasons) I think It is happening because of ati's crapy driver support because every once in a while I get a error that says it has to do with the display.driver file and I have all new ATI drivers. here is a quick run down of my specs.
AMD XP 2400+ 2.0
512 DDR 333 PC2700 RAM
160gb 7200rpm Western digital
soundblaster live 5.1
ATI Radeon 9700 pro all in wonder 128mb
sony dvd-rom/cd-rw
acer cd-rom
Windows XP home SP1
I also know its not a heat problem because my dorm is cold and I have 2 case fans blowing air correctly through my system and the problem has accourd in the morning 1 min after my computer was on (this is the time of day that my place is the coldest)
My advice to you is to turn off the auto restart feature and see if you get the same errors and problems as me... then we might be on to something!
In that case I would have to say that the drivers for the cards are making the computer act in odd ways which is crashing the kernal and so on.

