I deleted some file in regedit called "Classes" and ever since I deleted that, my computer won't get past the loading screen.
I can't even log in by using safe mode, so system restore is impossible.

Any help is appreciated, thanks.

repair your registry from the recovery console:

http://searchwincomputing.techtarge...

How to perform an in-place upgrade (reinstallation) of Windows XP:

http://support.microsoft.com/kb/315341

if you need a bootable disk to access your recovery console, download and unzip this ISO and create the CD:

http://www.thecomputerparamedic.com...

Boot off of a clean Windows XP cd ( your bios has to be set to boot from cd rom 1st ) choose Install ( Not > To Repair a Windows XP installation using Recovery Console, press R ) at the first prompt.
It will then search for a previous copy of Windows.
When it finds your copy, it will stop at the next prompt and ask if you want to do a repair. Hit R at this screen.
If it does'nt mention a repair > DO NOT continue.
It will then copy a lot of files and appear to be doing a reinstall ( But its not. All your programs and data are safe, all you will lose, are your Windows updates )

It says my product key is not valid..

If that is the exact error message, here is how you proceed.

http://www.google.com.au/search?hl=...