The .xls file is huge (over 4MB in size). The only code effecting the option buttons is as follows (each OptionButton has similar code).

If OptionButton18.Value = True Then
Range("d10").Value = 0
Range("d11").Value = 3
Range("d12").Value = 0
Range("d13").Value = 0
Range("d14").Value = 0
Range("d15").Value = 0
End If

Yes, the OptionButtons "sets" are all in unique Groups (properties box).

Do you think Excel Service Pack 2 would effect it?