To connect to your PCs: from the Finder, go to the "Go" menu and select "Connect to Server". If you wait for a couple seconds, your Mac will automatically discover the PC's in your network. After you select a PC to connect to, you will be asked to select the shared folder to connect to. This shared folder will then show up on your desktop just like another drive. So basically, it's the same thing as mapping a shared folder to a drive letter in Windows.
To connect to your Mac, you first have to go to the Mac, open System Preferences, and the go to the Sharing control panel. Select the "Windows File Sharing" checkbox to turn it on. Then you should be able to see your Mac from your PC's Network Neighborhood or Computers Near Me. I've seen other people in these forums complain about this but I have had no problems. The only limitation is that only your home directory on the Mac is visible. So if you want to share other folders (like a Music folder, for example), you have to create an alias for it and put it in your home directory. Works like a charm.
Hope this helps!