Win 7 networking

I wanted to ask this question of a Win7 group but couldn't find one. My
question revolves around networking together two WinXP systems and one Win7
system. Generally speaking, I can share files and printers between the two XP
systems. When I introduce the Win7 system and try to share its files out,
they don't show up on the network. In other words, the XP systems can see
each other's files fine but the Win7 system cannot see any shared files nor
can the XP systems see its files. Is there some trick to this? If I should be
posting this somewhere else, let me know. Thanks.