Make sure the correct workgroup name is entered in the Mac's network prefpane on the WINS tab.
In the sharing prefpane, go to file sharing and click 'options' Make sure that 'share files using SMB' is checked and that the correct account is enable for sharing.
Your Windows PCs should now see the Mac. If they still don't see the mac, and you're sure that all machines are operating on the same subnet, try connecting to the Mac using an IP address. If you can, then there's something screwy wrong with your workgroup setup. If you can't, then you need to look at sharing and your basic network configuration again.