Let's see if I'm understanding you correctly. You want to know how to create a directory on your slackware machine that is shared with both a Windows computer and another one with Slackware, right? The easiest way to do this, as you mentioned, is with Samba. Try looking in the tutorial section for a basic guide
to configuring Samba. If you want the share to be mounted on your 'remote' Slackware computer automaticly I recommend familiarizing yourself with smbmount. It mounts SMB/CIFS shares on a linux system and can be set uid root without comprimising your system much.