Windows XP starts then restarts over and over

Heaps of things can cause this - corrupt software, corrupt OS, virus, hard disk error. etc

First of all, I'd try turning the pc on, press the F8 key repeatedly until you get a list of options in dos mode text.
The first one listed will be safe mode.
use the arrow keys to select Last Known Good Configuration.
press enter, and, if you are in luck, your system will boot up just like normal.
This has fixed numerous pcs in the past.
If it works, I'd advise doing a checkdisk on the hard disk and running an AV scan, just to make sure.

If that fails, I'd again press F* repeatedly on boot, then select safe mode.
Wait ages, until you get a prompt with heaps of text, with yes to go to safe mode, no to go to system restore.
Select NO, and the system restore wizard will start... select a date previous to when the issue started, wait a while, and then test. if that goes thru, but the system still wont boot, i'd rerun system restore, but choose an earlier restore point.

If that fails, you'll need to boot from an XP disk, home or pro - the one that matches your OS - home or pro... boot from the cd, at first screen press r to repair via the recovery console.
select your os partition ( usually just press 1)
put in your admin password (usually just press enter, unless one is set)
and, when you can see the command line...
type in
chkdsk /r c:
and press enter.
this will check and repair, if possible, any faults on the hard disk.
once this completes, reboot and retest.

If that fails, you'll need to again boot from an XP disk, at the first screen press enter to install OS, press F8 to accept the licence agreement... then, if and ONLY if you get the option to press "r" to repair, press R, and allow it to repair the OS.
DO NOT CONTINUE PAST THIS SCREEN OR CHOOSE ANY OTHER OPTION OTHER THAN R, as you will OVERWRITE YOUR DATA.

To reiterate, if you don't get the option to press R to repair STOP, or otherwise say goodbye to your files.
you will not get the option to repair if you either are using the wrong xp disk, ie home disk with pro OS, or vice versa, or if the hard disk is corrupt, or the OS is too badly damaged to be recognised...

Hope this info helps.