Homegroups only work with other Windows 7 systems. If the other machines are XP then you'll need to change the *workgroup* name.
To change the workgroup go to Control Panel => System and Security => System and look at the "Computer Name, domain and workgroup settings." There's a link to the right that says "Change settings". From there look for the "Change" button. That will allow to to change the workgroup name.
Let me know if that works!