Do you see EXT (short for Extend) on the status bar?

If yes, try hitting F8 to toggle this setting. Some have complained that sticky
mouses can do this.

Try gently(!) banging on all the buttons--including the wheelmouse button.

Barb wrote:

Suddenly now when I click on a cell in Excel, when I move my mouse away from
the cell it starts to highlight all the cells in the direction I move my
mouse, even though I do not have the mouse button clicked to do this. It
won't let me out of cell selection mode and I cannot access the tool bars or
anything unless I Alt+Esc out and then use the Task Manager to close. Even
if I Alt+Esc out switch to another program, the cell selection still occurs
in the background in the directions that my mouse is moving in the other
program.

--

Dave Peterson