Re-installing office is a good start. You won't loose any work either.
Or this from a M$ search:
In Excel, the appearance of a number is separate from the number that is stored in the cell. In most circumstances, although it appears a number is rounding, only the display of the number is rounded. Calculations use the actual number stored the cell, not the number as it is displayed. You can change the display of numbers on the Number tab of the Format Cells dialog box (Format menu, Cells command).