Sounds very frustrating, sorry!

Do you have the Extend Selection enabled? It will display "EXT" in the
status bar at the bottom (Press F8 to toggle it on/off), at least in Excel
2003... I think 2007 still displays the status bar...

I have a co-worker who has a similar problem when his laptop is docked, the
left Shift key disables his extended selection.

Hope this helps, Jim
--
I appreciate any feedback.