I had the exact same situation running win 7 on all machines except one xp for legacy stuff. Tried almost every under the sun but no joy. I think it was something to do with the way win 7 (homegroup) does network protocols, but the only way I could get access was to dual boot the xp machine. It was weird even in xp (with win 7 installed but not active) I was able to share, access all.
Sorry I couldn't be helpful