Start here, be parasite free first.
XP is set to reboot on errors, shut this off and see if you can find out what is doing it.
Right click MyComputer -> Properties -> Advanced -> startup and Recovery -> Settings -> Find where it says "Automatically Restart" and uncheck it and select OK.
Now when it crashes, it will give you the blue screen with errors to search for.
I have been having a problem where my computer just out of nowhere shuts itself off with no error message or anything. The green light on the front to tell you its on is still lit up and for that to turn off you ahve to hold the power button for a min. i have a compaq presario 6315cl with 768 ram, ati radeon 9500 pro, soundblaster audigy ls. someone please help if you can!