I recommend doing a full wipe and reload, as anytime an OS downgrade or uninstall is performed it seems to cause trouble.
Make sure you have a Windows XP CD, and a valid product key, as you will need to activate Windows XP once it is installed.
Set your BIOS to boot from USB drive or disc drive.
Backup and Restore Windows Hardware Drivers
Backup and Restore Windows Activation Status in case you change your mind