If Windows XP reboots ad infinitum, it has encountered a Stop Error but is not reporting it to you. You need to see the blue screen of death (BSOD) to capture error details. Follow these simple steps.
Open the Control Panel and double-click System
Select the Advanced tab Click Startup & Recovery
Uncheck Automatically Reboot
Click OK, and OK again
Set the value of "AutoReboot" to 1
When you have made the changes, the next time a crash occurs, either note the error codes given to you on the blue screen, or inspect the mini dump file.
To interpret the mini dump file, read the KB article named How to Read Memory Dump Files.
Read this page on Stop Errors to cross reference the error codes and to obtain information on how to fix it.
If you cannot start XP at all, either normally or in safe mode, read this article called Cannot Start Windows XP.