BACK UP ALL DATA BEFORE DOING THIS !!!!!!!!!!! As it will format your hard drive.
Whenever you change the motherboard(MB) of a PC, be it any PC, Windows refuses to start as it doesn't like having the MB changed under it. It is something to do with licensing and copyright protection. The simple thing to do is find the restore disk that came with your PC. Turn on your PC, and open the CD drive, restart the PC and boot from the CD, if it doesn't do this automatically. This will re-install your copy of windows. During the installation it will ask you to insert the disk that came with your motherboard which it uses to install your MB drivers. When installation is complete you will have register Windows. All in all it should take about 20 minutes or so(Your actual mileage may vary)
Kezza
October 2009