Hey ningian,
So you want to completely wipe out your system and install a fresh copy of XP... If you have the install CD, pop that into your CD-ROM drive, and restart the computer... Make sure it boots off the CD, from there you will have the option of choosing if you want to format and install a fresh copy of the OS. It should detect the old install of XP (if that is what you originally had installed)... Just follow the on screen prompts... Should be fairly simple.
I would first back up everything you have that you want to keep before you flush your system, because once it is gone - it is gone...
Good luck,
-SlickVic78