PC becomes too busy is what exactly? Slow? I have run Windows 7 on fast Pentium 4 systems with 1 gig of RAM and it runs the same as XP on the same system.
To re-install XP, boot off the XP disk, run though the setup, you probably want to delete the partiton(s) that are there from the Windows 7 setup, if you don't have any data saved. If you do, backup your files, then install XP.