Re-installing WinXP Pro

What with the hard disk becoming more cluttered thanks to spybots, ad-aware stuff and the like, sometimes the best solution is a re-install of Windows XP on a clean hard disk (or partition).
What is the best way for the less painful way of doing this?
Bearing in mind all the re-installs of applications, with registration details to be re-entered….
Any help would be appreciated….
Peter