This is a very delicate problem because it offers many posibilities.

1. Usualy this happens when windows xp becomes corrupted and it will keep on rebooting itself and on rare ocasions it will boot but then crash for good.

Solution: This requiers a simple clean install of windows, after which everything will work normally.
Note: before trying the other steps we recommend to start with this one because is the most often of them all.

2. Problems with your memory cards. One or more memory cards could fail and your system will become unstable until those modules are replaced.

Solution: replace your memory module(s) with 1 from another computer which you know it works(from a friend,a neighbor)

3. Problems with your videocard which may have a failed circuit, thus resulting in a system instability.

Solution: replace your videocard with one from a friend just to see if it works with it.

4. In this category you will find the other posibilities that are very hard to detect and requier a PC expert to locate and repair them.

Here are: failed circuits, failed hard disks, failed motherboards, failed cpu's etc.

Solution: in this case we recommend you to go to a Pc service because trying to repair it yourself could hurt you or damage the computer.