Yes you can setup a PC to co-exist in a Mac network, if you're talking about sharing internet access. Without a server, what I can think of is manually assign static IP (use the reserved IP block) for every PC and Mac in the LAN. Then on the PC (with modem) you can actually have a proxy software and share out the internet connection.
On the other hand, you can always install internet-sharing software like WinRoute (www.winroute.com) and use the built-in DHCP Server and NAT system to share out the IP and internet access.
As for file-sharing and printer-sharing among PCs and Macs, without a WinNT/Win2K server, it's rather difficult (maybe third-party software) to implement.
If indeed you need the mixed network of PCs and Macs for internet sharing, then it should work just fine.
If you need extra help, do not hesitate to email me at auyongtc@tm.net.my
Best regards,
Au Yong