With a Windows PC you get either the manufacturer's "Recovery" CD which can only be used on that specific PC, or you may have bought a Microsoft retail boxed copy of Windows and installed it yourself (as I did). It's this retail copy you need in order to perform a "repair install". Alternatively you can perform a full re-install from scratch using either type of CD if you cannot get your problem sorted any other way.
Performing a repair install: http://www.geekstogo...ws-XP-t138.html
Re-installing XP from scratch: http://pcsupport.abo...nstxpclean1.htm
(Note to mods: couldn't find a 'pinned' post on re-installing xp from scratch, hence external link)
If the CD provided with your PC is labelled 'Recovery' you will need to use that instead, though it installs somewhat differently than using a Microsoft Windows CD. Your PC documentation should explain what to do but usually it's just a case of booting from it.
Edited by phillip22, 12 November 2008 - 08:39 AM.