Try a clean boot http://support.microsoft.com/kb/929135. If the problem goes away then it's just a matter of tracking down the culprit causing the problem. Follow the procedures in the article. Once found, delete, remove, deactivate, or uninstall it. Once done be sure to reset Vista back to normal status as explained in the procedures. If the problem occurs in clean mode then just restore the system to normal status and reboot - this solution is not going to work.
If that doesn't work and since you are using Windows 7, I suggest you repost your question in www.sevenforums.com
where experts in Windows 7 will be happy to assist you with your problem here (and any other problems or questions you have with Windows 7). Also mention that you're talking about the rectangular shape on the background so everyone is certain what you mean (assuming I'm right about what you mean).
Thanks and Good luck!
P.S. You will need to re-register on that site before you can post. Registrations here do not carry over to there.