Samba!
Mandriva Linux puts the samba suite in several separate packages, the main ones being samba-common, samba-client and samba-server. Ubuntu probably does something similar.
Samba allows you to mount a Windows share as a network disk and offer any directory as a share for Windows machines to connect to in the normal way, network neighborhood or network places or whatever MS calls it.
You can even setup a Linux machine as a domain server for Windows machines to connect to.
You can see more at the
Samba website.