copy whatever programs and files you need off the hd onto another hd or media.
remove the old hard drive from the tower, put it in new system. If it is the primary drive, ensure it is set for master.
Then grab your XP CD, put it in your CD rom. ensure your BIOS is set for boot off of CD rom first.
Boot windows install. Format your old hard drive and re-install windows on it. You will need to register your XP again. Bear in mind, you can only use XP ONLY on 1 system. Trying to install a second time will require a phone call to Microsoft to get a new key and proof that you don't have it on more than 1 system.