1) Have you made any recent change in hardware.
2) Make sure, processor Fan, SMPS fan, and cabinet fan is running smoothly.
3)Overheating:To check this cause..please open the cabinet and than run the system.
Please download speed fan .. it will tell you the temperature at which your CPU and hard drives are running.Download it from here:
*www.almico.com/speedfan.php
4)Another cause can be bad memory.There is chance that the memory(RAM) you have installed is bad.
To check this...try with another RAM stick.
5)There is a property in win XP..that restats the sysstem automatically when system is having any error.You can turn it off.To do this ,follow these steps:
start-->>right click my computer-->>properties-->>click on advance tab.You can see something like this:
*img45.imageshack.us/img45/724/propeg6.jpg
click on Settings in the Startup and Recovery section.A page willopen like this
*img119.imageshack.us/img119/6563/prop2zq8.jpg
Just uncheck the box "Automatically Restart" in the System failure section..like this:
*img253.imageshack.us/img253/572/prop2hi5.jpg
6) Another reason can be:: Virus/Spyware
turnoff system restore, boot in safe mode.
And do full system scan with one good antivirus and 2 good antisptware.Both should be updated.
7) Also repair your windows registry using some registry software.Some good and free registry fixer software can be found here:
*www.techhunt.org/node/94