I have had experience already and will pass it along. Buying a license for a Microsoft product without having the original disk and key # cert is worthless. There are tons of discount 'legit' keys on eBay and others. They do work. For three months.
In the days of XP, once a key was on, it was on. While a reload/repair or virus could mess it up, it would never turn off on its own. Microsoft could not remotely pull the key. With Windows Vista/7 it is all different. A key can work until Microsoft pulls it for being used to many times or for the wrong purpose. And they now do it all the time.
So someone will sell you an MSDN or other VLK key, and it will work. Then it will stop working three months later and you will have no recourse and you will have wasted your money.
CRAP! I see you mentioned XP as what you want. I will keep the above for the Windows Vista/7 information.
You still should get a disk and key on a genuine Microsoft cert to be legal since that is the reason you are doing this.