R help - Jul 2008 - Frequency-Table on Group Level - Multi Level Analysis

Hello everybody,

I've a multi-level structured data.frame.  

I = Individual Level (factor)
G = Group Level (factor)
SW= Software (factor)

I G SW
1 1 A
2 1 A
3 2 B

Im looking for a frequncy-talbe of the used software on the group level.
In this case the result has to be:
A = 0.5
B = 0.5

Is there an easy way to do things like this or do I have to read myself
into Multi-Level Analysis?

Thank you

Frequencey