In Control Panel of Windows 7, Network and Sharing Center, top left. Click change advanced sharing settings. Click the dropdown arrow for home or work.
Turn on network discovery. Turn on file and print sharing. Turn off public folder sharing, unless you want this for some reason. Use 128 bit encryption. Turn on password protected sharing.
Make sure that there is a password set and active for the Windows 7 user account. Reboot if necessary. Machines all should see each other now.
On Windows 7, start menu search box, type services.msc, hit enter. Make sure computer browser, workstation, network list service, network location awareness are all set to Automatic and are running.
Go on an XP machine and open a command prompt. Type net view, hit enter and see the goods.