SL randomly crashes after 10-30 minutes

Hi,
I can walk around in SL, but after a while, my computer simply reboots. Then on next start I get this message, if I want to send a crash report. I did not find any help in knowledge base about this. Only via google I found a forum where this problem was stated, and it was suggested, that it might have to do with nvidia cards. But there was no solution either.

My system is like this:
Win XP Prof
2GB RAM
Direct X 9.0c
NVIDIA GeForce 7600 GS (256MB RAM), latest drivers
some AMD Athlon dualcore (2x 2.5 ghz), somehow even aida32 cant detect the processor model and i dont recall it at the moment

any other information needed?

could be over heating. Sometimes that causes instant reboots.