Hamachi (free for personal use) from LogMeIn.com
It is cross platform and creates a VPN network between all the systems you setup.
You can then use any VNC or RDC software you like.
Since you are going Mac to Windows, I would suggests CoRD, an open source RDC client it is very good. I use it all the time at work.
If you want to stick with VNC, then any Mac client will work (Chicken, free RealVNC client, etc…) over Hamachi, even the built in Mac OS X Finder -> Go -> Connect to server -> vnc://windows.address.over.hanachi
I might suggest having TeamViewer or LogMeIn as a backup, in case there are issues with the primary.