First off, can you not even see any computers in the Network/Network Neighborhood?
1) What protocol are they using? (TCP/IP... netBEUI... ect)
2) Do the PC's have any drives/folders shared?
3) Maybe changing your workgroup to the same as the PC's might help?(Applications>Utilities>Directory Access>Unlock>SMB/CIFS>Configure>Workgroup)
4) Can only the PC's see the Mac but the mac can't see the pc's?
5) Can only the Mac see the PC's but the pc's can see the mac?
More info is better for us.