You will more than likely have blue screens of death and other problems unless the MB drivers are identical between the 2 boards.
You can sysprep your OS before you make the switch. To do this, go to C:\Windows\System32\Sysprep\sysprep.exe, check 'generalize', select 'OOBE' on drop down menu, and 'Shut down'.
When your new board/PC boots off of your old HDD, you'll get the 'out of box experience' screen, asking you to create new user account. Just call it anything and after that's done, you can log off and switch to your main user account.
Once you're logged in with your main account, you can safely proceed with deleting the newly made account in Control Panel > User Accounts applet
To clarify what sysprep does, it basicly gets...