VNC to Mac from Windows Displays Login window

I am sure that this topic has been covered before but I am having a problem with VNCing (if thats a word...) to a Mac from my Windows 7 machine.

I am using Real VNC and can connect to the Mac 10.7 but I am presented with a login screen even though I am logged in on my Mac. Entering the users password displays the same screen as the user sees but I don't want to have to enter the password.

I will be looking to deploy this in a corporate environment with users using Domain Credentials; I don't want Admins to have to ask the user for password y as some would not be comfortable with this.

Is there a way to VNC to a Mac without having to input the users Password after connection?

I have Apple Remote Desktop but do not want to have to Teamview to that then Connect to the users machine..