Shoutbox

Erm, if it was a mobo fault, you wouldn't even start  the PC (in most/all cases)

The problem is the fact that either you monitor doesn't support that resolution (not likely if you have used it at that setting before) or that the graphics card is not capable of displaying it (although I don't think it'd give the option). It may be a problem with the driver and the version of Windows you are using. Install any updates available from Nvidia (you may even need to do a lot of searching to get the right drivers [My default sound and other drivers are actually part of the Nvidia Ethernet drivers... go figure])