Back Up Activation Files. 
1. Open Windows explorer and navigate to C:\Windows\System32

2. Copy the files Wpa.dbl and Wpa.bak to another media floppy, CD-R or another hard drive (if you have more than one installed)

Now you can reformat your drive and reinstall Windows XP.
At the end of the installation, when Windows asks you to activate your product, decline and do not activate.

Restore your activation files.
1. Restart your system, and as your system boots up press F8 to access the boot options (Windows Advanced Options)

2. From the menu options select to start Windows in safe mode (minimal)

3. After Windows loads in safe mode, open Windows explorer and navigate back to C:\\Windows\System32\

4. If you find the files Wpa.dbl and Wpa.bak in this folder, rename the files to something like Wpa.dbl.backup and Wps.bak.backup

5. Copy your original Wpa.dbl and Wpa.bak from the CD or floppy back to C:\\Windows\\System32

6. Restart Windows
You should now have a clean and activated Windows XP