I found a hotfix for those who have XP machines on their networks that don't show up on the W7 Network Map.
What happens is the XP machine will show up at the bottom of the screen and can't be placed on the map correctly. It has to do with a protocol that Microsoft uses called Link Layer Topology Discovery (LLTD). It looks like it was first used in Vista. However it's not present or broken in XP.
The link below shows how to get a hotfix from MS to resolve the problem. I tried it and it works great. My home server which is an older XP box now shows up in the W7 network map right where it's supposed to.
Network Map in Windows Vista does not display computers that are running Windows XP