Unfortunately if you can't get into Safe Mode, there is no way out of the Automatic Restart loop For future reference, one of the first things you should do after installing Windows is to go to Control Panel>System>Advanced>Startup & Recovery>Settings and uncheck "Automatically restart". It is an idiotic default setting, that is only ever any use on servers.

For the immediate problem, you need to do an over-the-top re-install of Windows XP here. Boot up with the XP CD in the drive and let Setup run. Go past the "C" and "R" repair options by pressing Enter just as if you're doing a clean install and you'll eventually get an option to "Repair an existing installation". That's the one you want. If it asks about formatting you've gone wrong, so start over.

You'll get to keep your existing apps & settings, but you need to re-install SP2, Windows Updates & any unsigned drivers afterwards. NOTE: Be aware that you'll lose any protection from Blaster / Sasser type worms. Don't go online without a firewall, whether the built in Windows one or 3rd party.

How to perform a re-install of WinXP

Repair XP

WinXP Non-destructive Total Rebuild