By "seeing" do you mean ping or browse? Or either/both? I remember having some issues with my mixed network at first and having to go through a few things.

Under the Mac settings, do you have the Windows Workgroup set?

Under the Mac Sharing Settings there's Windows File Sharing and Personal File Sharing, I think I have both turned on on my Macs(Not home right now).

Can you ping any Windows boxes from the Mac?

Do you have any shares set up on the Windows PCs?