Don M
First off you see if you can even find a FULL RETAIL VERSION of WINDOWS XP, and as was already mentioned back up everything you need to save.
Put in the disc and follow the instructions, after the install go to microsofts site and download any of the updates.
Servette
first check you computer compatibility with XP
put the CD on d: and install it.
BACKUP YOUR FILES on an external drive or CD's.