does the xp machine see the W7? If you right click the network icon in your system tray, select network and sharing center, on the page that comes up you will see your network type, homegroup status, and on the left some options. select from the top set "change advanced sharing settings" if your pretty comfortable you can check out some of these options yourself or make sure everything is expanded and post a couple of screenshots well check it over. one thing to make sure is that network type is "home" not "public" also be sure that network discovery is on.
The networking setup in 7 is far superior to XP or even vista but is a little less user freindly due to the added functionality and security.Once you get it figured out you will enjoy it.