In addition to Captain Jack's articles, you may want to expain what the nature of the problem is.
Can you ping from and to the Win7 machines and the XP machines by both IP address and by computer names. That's the first step in determining if there is a software product in place that's preventing communications.
Additionally tell us if you are getting any type of error messages, like access denied or other errors indicating that the machine(s) are not available on the network.
Enable NetBIOS over TCP/IP in the properties of your network adapter (IPv4 properties, advanced button, WINS tab) also check the services console (services.msc) and make sure that the TCP/IP NetBIOS Helper Service is started and set to automatic.
Hope this helps...