Legally, Microsoft defines your motherboard as your computer. If you change the mobo, then it counts as an OS installation on a second computer. With OEM Windows, the OS will throw a fit. If you have Retail Windows, you can install up to five times, so unless you've already installed that same copy of Windows five times you'll be ok. Now, if you change your mobo because it failed and you had to get a new one, just call up Microsoft and they'll give you a code to re-activate Windows.