I don't think there's a limit to the number of times it can be activated. I think you can change your hardware EXCEPT for the actual motherboard itself as the XP OEM version is tied to your particular motherboard and also dies with it.
You shouldn't be able to, but in practice you still can buy oem copies of Windows without buying a new pc. Although it clearly contravenes the MS licence agreement, Microsoft's clarification of the oem situation has done little to stop the trade in single copies of oem software, often sold with no hardware at all. Presumably the vendor's defence would be that they thought they were selling to a small system builder?
Navid.s I'm guessing that you are considering buying this off EBay. My advice would be - don't. I know someone who did just this and, despite much effort, never managed to get it activated. Needless to say, the seller wasn't especially helpful.
I agree with you entirely, but I feel that it should be pointed out to whoever is thinking of buying one of these that both the sale and the installation are illegal. If caught, anyone (both buyer and seller) will be in big (and I mean big) trouble.