After your network cabling and homegroup is set up, the next step is to fine-tune it so that all the computers can find each other. If your network contains computers running different versions of Windows, put all computers in the same workgroup.
If computers running Windows XP are part of your network, it’s important to use the same workgroup name for all of the computers on your network. This makes it possible for computers running different versions of Windows to detect and access each other. Remember that the default workgroup name is not the same in all versions of Windows.
To find or change the workgroup name on a computer running Windows XP Click Start, right-click My Computer, and then click Properties.
In System Properties, click the Computer Name tab to see the workgroup name. To change the name, click Change, type the new name in Computer name, and then click OK.
In Windows 7, you could verify the workgroup by going to the control panel then click System. Click Advanced system settings at the left side of this window.
The System Properties will appear. You should be able to see the workgroup name used in your Windows 7 computer.
Basically, the key to adding XP computers to a Windows 7 network is to make it equal to the Windows 7’s workgroup.