Version: XP, Vista, 7
Verify your PCs are on the same network – wireless or wired – and all on the same workgroup (click Start, right-click Computer and select Properties; in XP you need the Computer Name tab). Is File and Printer Sharing enabled?
Check from the Network and Sharing Center in Vista/Windows 7 – ensure your network is Home or Work. In Windows 7 click Choose homegroup and sharing options > Change advanced sharing settings; in XP right-click a folder and choose Properties > Sharing tab.
Disable password protected filesharing in Vista or Windows 7 if sharing with PCs running XP, and check your firewall has placed your network in a trusted zone.