What network is your Mac on? Sometimes it's different to the one all your PCs are on. Go to "My Network Places" and check for a network called "Workgroup" it could be in there.
If everything is hunkydory on the Mac then your PC is the problem, it could take some rummaging around in all those folders to find the link to your Mac. You could also type the Macs iP address into the address bar on an Explorer window.