I won't have time to research this until we get home, but at the moment of messing with Office, the Security Essentials switched itself off, saying the reason was Windows not genuine.
That has happened to me many times. It should fix itself. It usually seems to happen after a boot. I'm on XP but I'm pretty sure that whatever bug causes it to fail to validate probably exists on Windows 7, too.
The real question is why MSE is constantly checking that to begin with. If it was genuine yesterday, it's still genuine today, after all.