I have a Mac Mini as a Home Theater PC, which is running Mac OS X Lion. And I also have a laptop with Window 7 Home Basic SP1.

I have tried to use Real VNC Viewer to connect to my Mac Mini before, but failed.

Today I tried again, by searching from web and found there is a solution now.

On Lion,

1. System Preferences->Users & Groups->Login Options, set “Automatic Login” to off.
2. System Preferences->Sharing, turn on “Screen Shareing” and click on “Computer Settings” button, set a password for “VNC Viewers may control screen with password:”.

On Windows,

1. Try to download “Ultra VNC Free Viewer“.

Then it will work for you. Many people said “Tight VNC Free Viewer” and “Real VNC Free Viewer” do not work well as the Ultra one.

Reference:
Apple Support Communities: TightVNC (or other windows VNC viewer) connecting to OS LION