Several users have experienced an issue in which Windows XP PCs cannot see Windows 7 PCs on the same network and/or vice versa. Fortunately, this issue is usually due only to configuration issues and can be easily fixed:
Change network setting to home or work If you have your group set to “Public” in the Network and Sharing Center in Windows 7, the system will be undiscovrable by Windows XP machines on the network. Open your Control Panel, then select Network and Sharing Center, and change the setting to either “Work” or “Home.” Make sure that all computers are using the same group.
Make sure Network Discovery is turned on In Windows 7, go back to the Network and Sharing Center in the Control Panel, and make sure that the “Network Discovery” option is set to “On.”
You may need to restart your computer after applying these changes.