another modem? do you have that other kind of internet? i cant remember what its called ... the kind that goes through a phone but isnt dsl ... hmm
but if you have the internet, just get a router and plud both computers into the router, plug the cable/dsl modem into the router and thats it.
file sharing is slick and easy, share a folder on the windows computer, then open the finder and go to network and it'll say windows network, bam there it is.
then set up a share folder on the mac and bam it will show up on the windows computer
at least this is what i did. maybe its just by chance that it seems to work perfectly, but i dunno