even if its oem you can still phone m.s. you will be put through to a rep and you just tell him that your upgrading... you dont need to say that your scrapping everything from the old rig and just keeping the hdd... just tell him you replaced the motherboard and needed to reformat due to a HAL. error. (hardware abstraction layer)
i did this for xp and my system activated perfectly when he game me a new number... problem is when ever you format it again you will have to phone m.s but this time all you will need to do is type in the number that windows gives you.