i have a windows xp professional desktop computer?

Pretty much what Stephen said on here. Only thing is on most computers when you start them up use the F10 key to go to the Recovery/Install mode. Choose to do a Recovery of the system. This will set everything back to factory settings.

I would replace XP with Windows 7, XP will dead by end of this year.

You need to OEM disk for the maker of your computer to re-install your Windows XP. Other machine OEM will not work because of missing drivers for your computer and your Windows key.

The WIndows key only works with OEM disk used by your computer maker.

Retail version does not work with your computer key because you have OEM key.

You cannot purchase new copy of Windows XP because MS pulled all copies off the selves.

Maybe you could find a copy on Ebay which would work with your machine.

You need to access the systems restore partition and set to reset the hard drive back to factory setting. Since you did not mention you computers manufacturer, you will need to check the manufacturers web site or user manual for the directions to access the restore partition.

get hold of a xp professional disc from a friend and use your cd key to activate or download a copy of the net

make sure you boot from disc not hard drive to instal

inother words press one of these buttons im not sure wich one continuously while you start your computer with your disc already in drive and change the boot order to cd/dvd drive first then hard drive

(F1, F2, CTRL+ALT+ESC )