Your O.S. X SHOULD come with Samba.
If not installed it should be on the install disk.
Then you need to configure it.
OK, the basics.
Install and set up Samba on the MAC. Share out the external drive in the samba configuration.
What is Samba? The whole world except MS uses TCP/IP, Transmission Control Protocol/ Internet Protocol for file sharing.
MS uses SMB, Server Message Block, now called CIFS aka Common Internet File System. Only it's only common to MS. The internet, Macs, Unix and Linux run TCP/IP.
Anyway, Samba is a TCP/IP to CIFS (SMB) converter.
Scroll down to the section on Darwin and MAC OSX
On the windows side mount the samba share. You should be able to do a search for remote devices and find it by the work group and name as configured in Samba on the Mac.
Except, isn't Win 7 Home crippleware? I don't think you can do remote mounts with that version.
But, I admit my Windows foo is very weak.
Once you have Samba set up someone here should be able to assist with remote mount problems if you have any.