Not able to switch between windows
 

This just started happening a couple of weeks ago:
When an Excel window is open (even if it is not maximized), it stays on top
unless I minimize it. Alt-Tab does not work, clicking on another program on
the Windows toolbar does not work, and even selecting a program that is
visible behind the Excel window does not bring it to the top. Does anyone
have a solution for this? I have found many references to this problem online
but no definite solution.

Windows XP SP2
Office 2007