Here is how to back up your windows registry before making any modifications, changes or hacks into it.
For Windows XP:

Click Start, then Run.
Type Regedit, press enter
Then click file, export
Name your file Regbkup.reg (file extension is .reg) and save it somewhere on your computer.

This way if you make any mistake with the modification, you can easily restore the registry using the backed up registry file For Windows XP.