Windows 7 has great networking features and is very easy to use, but there is some bad news: Windows 7 networking features are only fully compatible with Windows 7 computers. In other words, Homegroups are only going to work when they are set up between computers running Windows 7. Vista and XP do not support these features, so you’ll have to fall back on alternative networking methods.
That isn’t to say that you can’t network between different Windows operating systems. You most certainly can, but the process is a lot more involved and a bit less reliable. The difficulty of networking Vista and 7 isn’t terrible. You can’t use the Homegroup to control settings, but you Vista computers show up easily in the Network explorer view and network settings you select in Vista should apply when the computer is accessed by networked Windows 7 PCs.
XP is a different matter, and is much harder to network. However, BrightHub offers an in-depth guide that details how to perform various network actions when you are trying to network XP and Windows 7.