Hi,
As per Mathematics Rule, you cannot Divide any number by Zero. If you even do it on Calculator, it would return error. Similarly, Excel also Return Div/0 Error. To remove this error, you have to either enter numbers into the corresponding cells and the second number should not be zero. If you are working on such as sheet that you have to keep the formula as it is and don't enter the first value (blank cell), you can simple enter 1 to the second cell. this would return the formula result as zero instead of Div/0 error.
Hope it helps!
CreativeTECH

Hi I dont really have much to do with Excel as I essentially build and repair and install, gosh I still type with two fingers!! Now below I have provided a link to Microsoft . Ways to count values in worksheets etc. If the problem persists try to update Office with what ever sercice packs are required and see if this helps. the link is http://office.microsoft.com/en-au/excel-help/ways-to-count-values-in-a-worksheet-HA010287849.aspx and I do hope it helps.
Cheers