Sometimes the mouse buttons stick. Gently bang all the buttons (including the
wheel button if you have it) to see if that helps.

David McRitchie has some notes about this kind thing at:
http://www.mvps.org/dmcritchie/excel/ghosting.txt

Allanhart42 wrote:

How do I dissable this option? I select a cell and move the cursor which
selects other cells. I also cannot enter data anywhere or select any options
in the tool bar after slecting a cell. Extended Selection Option is dissabled
and verified through the status bar.

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Dave Peterson