If your intention is a complete clean install, you could take one extra step: start Windows Setup from the CD, and when it shows the partitions on your hard drive,
delete all partitions and then
exit from Windows Setup (press F3 twice). Now start again, creating a partition and formatting it NTFS. This is one way to prevent your boot-up from having an unwelcome screen showing two WindowsXPs to choose from (one of which is the ghost of your previous install).
Deleting partitions obviously erases all the stuff on them, so naturally you want to back up your stuff and verify that it's intact first.
If I'm misunderstanding, and you're planning a repair installation over the top of your existing Windows XP, here's Microsoft's info on how to do a repair installation, if that helps:
http://support.microsoft.com/kb/315341/ I'd prefer to do a full installation myself.