Did you download and install that utility I posted? It will let you know if your PC is running hot.
The other thing you can do is re-enable blue screen errors in Windows XP instead of allowing an automatic restart.
- Go to Start and Control Panel
- Make sure you're in Classic View.
- Select System.
- Go to the Advanced tab.
- Under Startup and Recovery, click Settings.
- Under System Failure, untick the Automatically restart option.
- Click OK on all dialog boxes
Hopefully that should give you an error message if the system locks up or becomes unstable, rather than automatically restarting your PC. If so, you can make a note of the error message and then go from there. Try installing and running that program I posted first though.