This situation sounds very similar to an infection and MBAM may not necessarily locate every virus/malware out there.
I would try installing another utility, such as SUPER Anti-Spyware, and scanning with that as well.
The sudden nature of the issue means it is probably virus related.
Although unlikely it may be a hardware issue.
Bad hard drives can cause similar issues, but they tend to go bad more gradually, unless they receive external damage.
Edited by akaUriel, 17 July 2012 - 04:32 PM.