My computer is a Dell Studio 1747.  I ran Dell's system diagnostic and everything passed.  I thought it might be a stuck key, so I removed the internal keyboard and connected a USB keyboard.  No change.

While testing the keyboard, I figured out the exact action that causes the problem to occur.  When I start Photoshop, it works correctly until I use the keyboard for anything or switch to another open application (using the mouse).  Then the problem occurs and stays until I restart Photoshop.  This happens using the internal keyboard or with the internal keyboard removed and using a USB keyboard.  An example:  I use the Rectangle Selection tool to select an area.  This works repeatedly.  Then I press and release the Shift key.  Then I try to select an area.  The problem occurs and the tool no longer works and stays that way until I restart Photoshop.

No keyboard macro programs installed.  Microsoft Security Essentials is installed.  It says it is up to date and there are no problems.  A keyboard test program found no problems. I have tried two USB keyboards with the same results  I have 10% free on my system drive: and 15% free on my second drive.

More clues:

• If I open Photoshop by opening a file, the problem immediately occurs. If I open Photoshop and then open a file, it works until I press a key as described above.
• The problem only occurs in Photoshop. I’ve tried lots of other programs, including Premiere, Audition and Illustrator; they all work correctly.
• The problem can occur without a keyboard attached.  As I described above, if I switch to another application (Windows Explorer in my test) and then switch back, the problem occurs.  This is true even if there is no internal keyboard and no USB keyboard.