It's hard to say without knowing a little more. Do you have the cell formatted as a percentage? If so that could be one reason.
There are two other possible things I can think of;
Go to Tools > Options...
Then under the Calculation tab see if Iteration is checked. If it is, then uncheck it.
The last thing I can think of is if somehow your computer settings are messed up rather than just Excel.
Go to your Control Panel in windows and select the Regional and Language Options. Select the Regional Options tab on the window that comes up. To the left of where the customize button is, English (United States) should be listed, provided you live in the US. If that is not selected, make that your selection. If it is, then click the Customize button.
You should have the following categories and values under the Number tab:
Decimal symbol: . (period)
No. of digits after decimal: 2
Digit grouping symbol: , (comma)
Digit grouping: 123,456,789
Negative sight symbol: -
Negative number format: -1.1
Display leading zeros: 0.7
List separator: , (comma)
Measurement system: U.S.
Standard digits: 0123456789
Digit substitution: None
Under the Currency tab you should have:
Currency symbol: $
Positive currency format: $1.1
Negative currency format: ($1.1)
Decimal symbol: . (period)
No. of digits after decimal: 2
Digit grouping symbol: , (comma)
Digit grouping: 123,456,789
The commas and periods I added the parenthesis part in case you had trouble distinguishing the two. (comma) & (period) are not part of the values. These are what my settings are for windows with my Excel working fine. So you might consider changing your settings to the same if any of the values are different on your computer.
There is one last thing that I can think of. It may or may not work. If you have XP, you could try doing a restoration to an earlier point. Restore to a point before you started experiencing problems with Excel. A restoration can be undone, but I don't like doing one unless I have to.
If you still have trouble after that, it might be a good idea to call the Microsoft help line.