Why not try TeamViewer.com, which is cross platform.
To use a 3rd party VNC client, you have to configure
System Preferences -> Sharing -> Screen Sharing -> Computer Settings -> [X] VNC viewers may control screen with password: <provide your password here>
This should allow your PC based VNC clients to work with Mac OS X Screen Sharing.
VNC clients will connect to the logged active user.
However, Mac OS X VNC server has been known to be picky about talking to 3rd party VNC clients, so you may need to try several. Also you must allow the Mac OS X VNC server to specify the screen resolution and color depth. Some VNC Clients give you the option to use fewer colors, but Mac OS X Server does not play nice with reduced colors.
Another option would be to disable the Mac OS X Screen Sharing option, and install a 3rd party VNC server, such as the "Vine Server"
Vine Server (aka OSXvnc)
<http://www.macupdate.com/info.php/id/11283/vine-server-%26-viewer>
which does play nice with 3rd party VNC clients.
But in all truth, I would look at TeamViewer.com as it is very robust and I have found it easier to use in many situations where I do not want to play cross platform games with Mac OS X built-in apps and services.