desktop changed

My desktop was changed back to its original out-of-the-box background as of this morning. I don't recall doing any updates yesterday. Also, when I opened Firefox it seemed to think this was my first time & asked if I wanted to set it as default browser.
A search shows that others have had this issue with older versions of Windows, but not with mine, which is Windows 7 Home Premium on a Sony Vaio.
I ran my AVG free scan and it found nothing amiss. Should I be worried?
Thanks,
P'lite:confused

 

Out of curiosity what is listed under: c:\users? I'm thinking it could be a corrupt profile. Windows sometimes will make a new one.

 

The problem corrected itself after I restarted it. I looked under C:\users and found folders All Users, Default User, Me, etc. I didn't see any folders that were created recently. I guess it was a just a blip so I've decided not to worry.
Thank you for your reply!
P'lite

 

Run sfc /scannow from a Command window, to check for possible problems.

 

I ran it and it said "Windows Resource Protection did not find any integrity violations." Guess it was just one of those things.
Thanks!