Can you use procedures?
Code:
Local MyVariable
MyVariable = sum(jan)
Do Case
Case MyVariable = 0
....
Otherwise
...
End Case
It's hard to find a solution here without knowing more about the problem.
Is sum(jan) a calculated field?
If so, what is the name of the field it is calculating the sum of?
I doubt the above will work, but give it a try

(it's been a while since I did reports in VFP!)