[Gareth]

I agree with JohnHorb, before going so far as reformatting and/or system restores, check your Event Viewer. Windows can sometimes be useful and helpfully log an error as it occurs.

Try and cast your memory back to when it last occurred, and then see if there are any errors listed that correspond to that time, as per JohnHorb's instructions.

If there's nothing, then you can investigate further, but this should be your primary source of investigations.