As "Act of Kindness" has stated the best way is clean installation of XP. However you can buy and use the upgrade version but when asked/prompted during the wizard alter it from Recommended to Full Installation which re formats your hard drive alter the file system to NTFS. Also do not forget to back up all your data before starting as you will loose it..
and buy the up-grade version, it is exactly the same disk. You can still do a clean install with the only difference being, that when you start the install, you will be asked to insert your qualifying CD.
Pop it in and it will take a couple of seconds to check, then replace your XP CD, and the install will continue. J.