In your Control Panel -> Network and Sharing ... Does it say "Home Network" or "Work Network" ?
Home network is strictly for ALL win7 networks. XP, Mac, Linux etc. aren't compatible or welcomed in HomeGroups.
If you want to network with Other OSs you will have to set your Win7 machines up to use "Work Network" (Workgroups) and "Username and Password" file sharing. Then you will have to create your file shares the old XP way, even on your Win7 machines.