Browsers keep moving off desktop randomly

Okay, this is a problem my father is having with my mid-'09 MBP that I gave him to use. Today I finally got to witness the problem firsthand. He had originally told me it was a Google Chrome problem but I have found that the same problem occurs in Safari as well. He also has an '08 iMac which doesn't have the same issue. Pages in both Safari and Chrome will randomly "minimize" to the top or sides of the desktop. This occurred 5 times in 3 minutes this morning. It seems to happen only when the mouse is moved after sitting idle for short periods of time. Needless to say it is a very frustrating problem but not one which makes internet browsing impossible. Below are attached screenshots. Note the barely visible edge of the Safari window underneath the top of the screen. Clicking on this edge will restore the window but only briefly before disappearing again.

this what happens when you press Cmd + Expose together.

Are you using a tool like "bettertouchtool" with this function set as a macro?

No, no macros or anything like bettertouchtool. I thought it might be a problem with the mouse itself but replacing the Microsoft wireless Mobile Mouse 6000 with an old Logitech M700 (wired) does not fix the issue. The slightest touch on the mouse causes the aforementioned problem. My Dad's computer knowledge is limited to Startup, browsing and shutting down (no offense Dad if you read this).

Show Desktop can also be triggered through a hot corner. Go to System Preferences > Desktop & Screen Saver > Screen Saver > Hot Corners. See if any of the corners are set to Desktop.

Negative again, here is a screenshot. Oops, never mind that is exactly the problem. Thank you sooooo much.