I just recently upgraded to Leopard, so a lot of the things that are different about it (compared to Tiger), I’m just now finding out. One of the things that apple changed was the name and location of Windows Sharing in Preferences. It is now called SMB and here is how you find it:

Start off by opening up “Sharing” in “System Preferences”

Then, after you have “File Sharing” checked and highlighted, click on “Options…”

From the drop down menu that shows up, check the box that is called “Share files and folders using SMB”.
Though I’m not entirely sure why Apple calls Windows Sharing SMB now, this will for sure get the job done.

Your Mac (running Leopard) will now successfully show up on a Windows Computer! Yay!