SQL Select Distinct with MS Access 1

Hi there,

I have a table of data similar to this:

staffId weekNumber hoursWorked
======================================
1 24 41
1 25 12
2 24 36
3 24 29
2 24 39
4 25 18

Each row represents a shift. What I need is to find out how many weeks a staff member has worked for us for. I think this would work:

SELECT COUNT(DISTINCT weekNumber) FROM tblName WHERE staffId=3

However Access does not allow COUNT DISTINCT. I have tried everything I can think of, can anyone shed any light?

Thanks!