I have never had a problem re-activating an OEM version of XP Home or Pro.
Yes, you may have to call Microsoft's 800 number to get a new activation code, but this has never been a problem if your situation is "on the level" (i.e. a hardware upgrade, system overhaul, or similar single-serial + single-PC).
Heck, I've totally overhauled an OEM XP Pro COA, then re-activated it on completely new hardware. I was just a hassle-free call to Microsoft's 1-800 number, answered one question ("Upgrade the PC") and they gave me the new number to punch in. Total time: 1.5 minutes.
If you've activated the same serial more than 20 times in the past year, yes.. they do get grumpy with this.. but still, even in this rare scenario (my boss did this to his PC lol).. they still just fork over the new activation code if you grovel a bit. lol.
I believe the only exception to this is specialized add-in OEM versions of XP. Some COMPAQ's or GATEWAYS do NOT use a Microsoft OEM build of XP, but instead a very customized version with several core components highly modified. I do not believe Microsoft *can* re-activate these and it's up to the support depo of your OEM system with such custom XP builds. In this case, you're often times better off just hunting down the same series CD (look at your COA for what XP "build" it's based upon, like X08, X09 or X10 - XXXX on the COA sticker) and installing this versus the custom, OEM tampered build.. then crossing your fingers driver support is solid enough for plain-jane XP.
