How do I access Windows PCs on the network?
Sharing must be enabled.  To see the Mac from your PC's, enable File Sharing (System Preferences/Sharing/File Sharing).  You will need to take the additional step of enabling Samba sharing, since PC's don't use Apple's File sharing Protocol (AFP).  Do this by then clicking on options, and make sure "Share files and folders using SMB (Windows)" is checked.  Turn it on for whichever accounts you wish to authorize.

In order to see PC shares from your Mac, the PC's must be on the same network, with sharing enabled.  Share a folder or something.  Is it XP, Vista, 7?  If it's XP, disable Simple File Sharing.  Do so by opening a folder, going to Tools->Options, and find it at the bottom of View.  Then right-click a folder, go to Sharing and Security, and enable sharing.

If it still doesn't show up, you may have a network/firewall issue, and it warrants further help.. but try that to get you started.