I've discovered an interesting, and extremely annoying issue with 2007.

During construction of a worksheet, certain cells are changing their format
as I enter data into them. For example, a cell will start as "General"
numbering format, yet as soon as I type in a number, (say, 7,) the cell's
format is changed to Percentage, and automatically adds the % at the end of
whatever I typed in. Changing the format back to General results in the
number that I had originally wanted to enter, but divided by 100. How can I
stop Excel from changing my cell formats while I am entering data?