Yeah, its completely possible.
At home, i have pcs and macs.
All my macs are running Mac OS 10.2.4 and one Mac OS 10.3 beta. These OSes are compatiable with networking with PCs with no additional software, it has it all built in. These Macs can share files/receive files from windows machines. All you need to do is hit: Apple + K and it will search the network for all computers that are sharing files, including PCs. if you want to share files, go to: System Preferences -> Sharing -> Click on "Share with windows machines" (or something like that). It should work that way..
If you have a mac thats running, Mac OS 9 and below you need additonal software to communicate to Pcs. Software like DAVE or MacSOHO will make the Macs and Pcs communicate easily...
I hope this answers ur questions

thanks, paul