My Windows 7 automatically ocassionally reverts to an earlier state

My Windows 7 automatically ocassionally reverts to an earlier state

My Windows 7 Professional, service pack 1, 64 bit, often reverts itself to
an earlier time. I notice this by files and icons that I have deleted from
my desktop suddenly reappearing. Looking through my update history, I
notice that no updates are logged since March 2013 (this is August). I
have auto-updates turned on and I know updates have taken place.
It acts like a system restore has taken place, but I didn't do it, nobody
else has access to my computer and I am not on a network. It seems to be
resetting itself. The worse thing is, when I restore the system to a time
before this happened, it does not change it back to the way it was.
I have considered the possibility that it could be a virus or something,
but I have a paid version of AVG Internet Security 2013 (up to date),
although I realize virus protection isn't always perfect. I have done much
research, but can't find any answers.