If you have OEM written on the Product Key sticker (that's the one with your windows licence number) you will not be able to use it. OEM licences will not work on another PC. As you have changed the mobo the OS will see this as a different PC.
Also, the old Windows installation will have a different set of chipset drivers installed on it that would have been for your old mobo.
Your best bet will be to run a clean install on the hard drive and then reinstall the new mobo drivers. (You should have a disc that was supplied with the mobo)
Finally, if the hard drive is SATA you need to get the SATA/RAID drivers from the mobo's manufacturer's site and put them on a floppy to install using the F6 key when the message appears at the bottom of the screen early in the installation process.
