For the first time in 20 years of using Excel, I am finding that auto
calculation is not taking place whenever I add new entries to a range of
cells included in, for example, the SUM function. I only get the correct
results after invoking the function again. Even to calculate a fraction, the
correct answer is not automatically updated if the numerator or denominator
changes. Yesterday, I emailed the worksheet I was working on to my office
neighbour's computer and I was able to edit the sheet and get the updated
results automatically.

Could there be something wrong with the version on Excel 2007 on my PC?