I have had a full retail (not OEM) version of Windows 7 x64 Ultimate installed on a PC for about a year. The SP and all of the many updates that MSFT has issued in the interim have been applied. If the boot drive is transferred to another PC with a different motherboard, as its boot drive, would the SP and all of those updates have to be reinstalled?
The service packs and updates are still in the OS itself. Changing motherboards is like changing hardware. I've had varied success doing this kind of move, but my most recent time was with Windows 7 and it was quite smooth. In XP, I'd do backups, sacrifice some Jolt, and cringe when powering up. Sometimes it would work and sometimes need a re-format.
You might lose your service packs if you end up running the 'repair' process to get the new machine to boot.
Avoid it by making sure the OS is ready to move to new hardware.
The most important thing to do is prepare the OS before you move the HD.
Use sysprep as outlined in http://windows7easy.wordpress.com/2011/01/25/transfer-your-windows-7-installation-to-a-new-pc/