I too have a similar setup to yours. I found that Vista would not list the shared XP folders even though they were all on the same network. I discovered this update that needs to be run on the machine with XP. I did it and all shared files on XP became visible in Vista.
The hyperlink function on this Bulletin Board makes no sense to me so here's the address:
http://technet2.microsoft.com/Window....mspx?mfr=true