Wayne's right, OEM is just what they send to manufacturer's who build computers, etc. It's the same program.
And yes, anytime you have any slight hardware change - as in a component, not a fan or accessory - Windows want you to reactivate. If you've already activated it once, you'll need to call a number. You give them the number on the screen (voice automated) it directs you to a customer service person in India and they might ask you questions like are you putting this on more than one pc.. etc. Then they'll give you a number, you punch it in and XP's happy.
Just they're way of tightening control on their monopoly.