Unable to change background Windows 7 Home Premium

**Original Title**

Windows 7 Desktop Background Picture won't change. Was working fine, changing everyday, then one day, stopped changing and now stuck on the same picture for 2 weeks.

The desktop backgrounds were working fine (previous 2 weeks ago), picture would change everyday. I downloaded all the other themes (ex. Brazil, China, Spain, etc.). At first these other themes worked fine and then all of a sudden, the desktop background picture won't change.

Another problem: under "Control Panel > Appearance and Personalization > Personalization" window, when I click on "Desktop Background" link or icon on bottom left and the picture locations are displayed, I can't check or uncheck pictures. When I try to clear all check marks or select all check marks, they seem to disappear (the check marks) randomly and then reappear randomly. Sometimes some check marks will stay and then others will disappear despite selecting "Clear all" or "Select all" (doesn't matter which) and seemingly random pictures are selected or deselected in no particular pattern. And then I tried manually selecting and deselecting each picture check mark and same thing, check marks would randomly stay or else disappear.

Bottom line is I am unable to change anything in "Desktop Background". And I have had the same background picture for 2 weeks unable to change it. Have clicked on every button or drop down menu like "Change picture every:" to 10 seconds, 30 seconds, etc. and still nothing.

I tried finding a work around solution like right clicking on different pictures in Windows Explorer and selecting "Set as desktop background" but literally nothing happens. I tried viewing several pictures in "Windows Photo Viewer" then right click on "Set as desktop background" and then an error pops up "This image can't be set as wallpaper. An internal error occurred."

I have all the latest windows updates. I haven't tried anything in safe mode because I really don't know windows 7 and safe mode turns the desktop background off, anyways, doesn't it? Don't really want to do a system restore for something so small or re-install. Please advise other things I can try!

Should also mention I tried deleting all the additional themes I downloaded to see if these caused problems... and yes I used antivirus and spyware several times (using "Shaw Secure" made by F-Secure). Still same problem. Also scanned with Ad-Aware (although had to turn off F-Secure to do this). Still nothing wrong.

It's a small problem I can live with but also annoying.

Should also mention this is a brand new pc with Windows 7 installed on it (no other OS was installed on it).