I left my Windows 7 x64 PC to do a backup overnight and when I looked at it in the morning, there was a pop-up saying I should go online to find an antivirus and enable windows defender. Turned out that my Windows Security Essentials 2.0 was no longer running and after a restart, all traces of it were gone. I had it installed for several months now and did nothing to uninstall it. I reinstalled it, but it would be interesting to know how something like this happens, particularly to an antivirus program. The computer did not restart during the night, so this shouldn't have been due an update (last update was installed on July 22).
UPDATE: Since I can't answer my own question, here's what I just found in the event log:
Description:Cannot complete the Security Essentials Upgrade.
Security Essentials is not currently monitoring and helping to protect your computer.
Please restart your computer and try again. Error code:0x80070643. Fatal error during installation.
So hopefully this wasn't a virus.