You need to use Samba to talk to windows machines (there are other ways, but samba's by far the most convenient). This bit can be either really easy, or pretty involved(depending on your distro), and if you want it set up correctly (for security and efficiency), it pays to spend some time learning it. You'll need to install samba and the samba server (may be one package on some distros), then read through the documentation to do a basic setup, and adjust it from there to your particular needs.
This should be a good starting point: http://www.samba.org/
has a good number of documents as well. At first glance, even after reading the docs at these sites, samba configuration looks next to impossible for most people, but in reality, to get a working configuration, you only have to edit a couple lines in your /etc/smb.conf file, and the file itself contains enough comments to help you out. It looks much scarier than it actually is, something that seems to hold true across the board for a lot of Linux stuff.
Oh, and something newbies tend to overlook: You need to use smbpasswd to set a password for your clients. "smbpasswd -a username" should do it, assuming you adjust security later.