Disable Opened App Reverting to Desktop When I Move the Mouse

New to Windows 7, but so far I'd say it does have some nice enhancements over XP.
Having said that, it does this incredibly annoying thing, when I'm working on an open app, such as Microsoft Word.
If i happen to move the mouse to the right, Windows opens up the desktop, covering up the work I'm doing.
To get back to my work screen, I have to move the mouse back to the left, I believe.
This didn't happen in XP.
What is this and how do I disable it?

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