We have the same problem here with some of our spreadsheets.

when you click on one cell excel highlights the cell you seleted and 3 or
more below it in the same row! editing works as though you only have one cell
selected.

it gets worse though!......

if you zoom in a bit you select more cells with each click - zoom out far
enough and it works fine - you only select one cell with each click - i've
spent some time trawling the net to no avail.

It doesn't seem to affect new wokbooks only ones which have been knocking
around for a while - but i don't feel like starting from scratch with the
hundred or so template files we use here!

I am yet to find any acklowledgment of this issue from microsoft.

F8 and mouse drivers and office 2007 SP1 seem not to make any difference
whatsoever.