if u change your mainboard a hundred times, you'll just have to call MS to activate your system.. thats all.. you can still use the OEM key.. this has been going on for my review system for a long time and i can remember all my key presses for my CD Key.. the only problem is that sometimes it takes a long time as you have to press all your keys and listen to the automatic voice repeat it..

For XP
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Call hotline..
Follow instructions..
Key in CD Key..
MS rejects your CD Key..
Operator picks up call..
repeat CD Key..
Gives you a phone activation code..
DONE..

its actually almost the same with the retail version, just that you can try a few more times more than the OEM..