As you already know, Win 7 & vista stores documents in a different home folder as XP. so all you need to do in Vista/Win 7 is add that XP home folder to the list, and it'll be merged in the list.
In Win7/Vista, click Start, Right-click on Documents, Left click Properties. You'll see the folder already there from Vista/Win7's new home folder {c:\users\<username>
Click on Include A Folder, browse to where the XP documents are located ?:\documents and settings\<user name from XP>
So, Say the XP username was Hawk, and you did as above. when you go into your Documents folder, you'll see a folder called Hawk. That'll be the old (XP) home folder.
N.B.: It does NOT automatically copy/move the files from the XP folder, it just lists the contents of both folders in Windows Explorer and/or Homegroup.
