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#11
The XP systems can't join a Windows 7 homegroup because they don't have IPv6, which is used for homegroups, but you should be able to share any folders you can see from other computers.
In order to share the folders, you need to mark them as "shared" in a specific way on the systems.
Here's what I do: In Windows 7, right click on the folder, go to sharing, click on the down arrow in the box and set it to shared with everyone (you can specify a more restrictive set of users if you want). Then click "Share" at the bottom, then done.
Next, go to "Advanced sharing" in the properties, and click on "Share this folder". (The default name will be that of the folder. You can change that by clicking on "Add".)
Finally, click on the "Permissions" tab at the bottom and change/set the permissions for the folks you selected to share with - the default is read, and that is probably best, unless you want them to be able to modify/delete your data.
On XP, you also have to set the folders you want on that system to shared. It is a similar process of right clicking on the folder and setting the share permissions.
At the end, you should be able to see and share data between all the systems.