Gary Ralston describes a cross-platform, screen-sharing system he worked out, using VNC:
It took a while, but I found a workaround with VNC for a Windows machine to screen-share my Mac running Mac OS X 10.3.5. While free, the solution is inelegant - you have to set everything just so to get the connection working.
On the Mac, we ran Share My Desktop 1.2 . On the PC, we ran TightVNC.
On the Mac:
1. Set the Mac to 1024x768, Thousands (256 or millions will fail)
2. Launch 'Share My Desktop'
3. Enter port 5901 (VNC uses ports 5900 and higher, one port for each monitor, as far as I can guess)
4. Enter a password
5. Press 'Share My Desktop'
On the PC:
1. The PC seems less sensitive to bit depth and resolution settings. We got it to work with 16 and 32 bit at 1024x768.
2. Use Tight VNC client - try fast and best compression versions.
3. In Tight VNC preferences, reduce the default compression to get an acceptable picture (we used a setting of 5 with good results between machines connected via cable modems. 9 is pretty pixilated.)
4. Address the Mac using the public IP address, a colon, and '1' (so 22.214.171.124:1) (This works fine behind a router - forward public port 5901 to the local IP address of your host Mac)
I've done something similar. Microsoft's RDC is much better for controlling a Windows machine.