1) Click Start, Control Panel, System, Advanced Settings [third one], UN-check the box next to "Automtically restart". When XP decides to restart you'll have a blue screen, this screen will tell you what is making XP cause you problems. Post back what the info you see.
2) Click Start, Run, Copy and Paste:
Click on APPLICATION and look for RED errors, double click the log and within this log there should be a link to microsoft site where it may or maynot tell you of the problem and the cure.
Do the same for SYSTEM.
3) Click Start, Run, Type:
Click STARTUP and remove all ticks from the boxes and restart your computer. If the computer restarts etc then its not a software issue. If the computer decides to work for you then I suspect a software problem. To find out which piece of software is causing the problem. Stick a tick back in the bxes [one at time] and it should reveal the troublemaker.
4) Click Start, Control Panel, System, Hardware, Device Manager, are there any exclamation marks etc?
5) Click Start, Run, Type:
Click Components, Problem Devices, anything shown?
Have you tried a new clean install of XP?