Here is an easier method to work around connecting to XP Machines under windows 7:
Open network and sharing center, click change advanced sharing settings, Turn off password protection to make it easier while connecting with other windows 7 machines, and make sure file sharing is enabled... now here's the key part, windows XP will not except blank passwords for network connection. All you need now is to set a password for the user account that is being used. The network group isn't a factor here at all, you can use \\*IP Address or network name* (example \\192.168.0.100 or \\*username*-PC) when you click the start menu and run or the search for windows vista and 7. If you wish to share folders under windows 7, go to the folder you want to share and right click and properties to get to the security tab and make sure to add users in the permissions or even more simple, add *Everyone* and set permissions that way. Now you shouldn't have any issues with file sharing with other OS's other then windows 7.