Connect to VNC from Windows
If you use Vine Server VNC as the server on the mac and open up port 5900, and VNC Viewer on the PC (or any other vnc viewer) you'll be fine.  Its not native, but its more flexible.

This, of course, is a free solution.

See this thread on mac screen sharing:


Controlling a Mac remotely


Vine Server here: 


http://sourceforge.net/projects/osxvnc/


UltraVNC (client viewer only): 


http://www.uvnc.com/download/1082/1082viewer.html