This problem has been driving me batty.

What happens is this: I open Excel and start working, and everything is
well. I can use my wheel mouse button to scroll up and down (vertically)
throughout my worksheets with no problem. Then at some point during the day,
Excel will decide that when I spin the wheel mouse, it wants to scroll my
worksheets from left to right (horizontally). I can still scroll vertically,
but only if my mouse pointer is within about an inch of the vertical scroll
bar on the right. If my pointer is in the middle of the sheet, it will
scroll horizontally when I spin the wheel mouse. This drives me nuts, and I
need to find a way to disable the horizontal scroll feature.

I'm running Microsoft 2003 Professional Edition, and this has been happening
ever since my IT folk installed it on my work computer, maybe about three
months ago (was previously running Excel 2000). It seems that the only fix I
have found is to completely shut down Excel then start it all back up again,
but even then it will start happening again eventually. I've looked through
all of my mouse settings and all of Excel's settings, but I can't find
anything that will keep this from happening.

One thing to note: I typically open a new window of my workbook and then
arrange the two windows horizontally on my screen so that I can view two tabs
at once. I'm not sure if this has anything to do with it, and I haven't
noticed the problem starting when I open the new window, but I thought it
might be worth mentioning.