I don't have enough info from your post to know, but I can give you a couple of guesses. It depends on what kind of OEM install CD you have. If it is one supplied by a white-box system installer or purchased from a retail store, then you will have 30 days to activate your installation. (The CD key is only part of the process.) In this case, you won't see a message at logon telling you you need to activate until about three weeks have passed. You'll have about a week of use from your PC from that point before you will be locked out of Windows until you activate -- or re-install to a freshly-formatted drive. If you have an OEM CD that comes with the systems built by Dell or HP or Gateway, etc., then those CDs will only work with the original motherboards without reactivation, as I understand it. In either case, changing the motherboard is considered changing the computer. If the OEM copy was previously installed and activated with that CD key, Microsoft will expect you to buy a new copy of Windows. --Joel