set up bios to boot from cd, then boot run the installer chose the location(the same as the old os). then chose to format that drive, problem solved. then you can do a clean install.
According to Ms site, it can be found in add/remove or you can use the XP disk by using the recovery console or alternatively the hidden $ntservicepackuninstall$
see http//support.microsoft.com/kb/875350
I have the full version of XP Pro 3 which i want to use for a clean install
If you have a Full XP pro CD. Boot to it, when it gets to the harddrive section, delete the partition, create a new one, format( if you know your drive is fine do a quick format) and install. You have a clean install of XP.