I ran into this last night on a new install of Windows 7 64 bit, with UltraVNC 188.8.131.52 as server
I tried setting the winvnc.exe file to always run with Administrator privileges, and it seems to have worked.
Go to your UltraVNC installation folder (usually C:\Program Files\UltraVNC and look for "winvnc.exe" (if you have Windows set to hide known file extensions, then it will just show as "winvnc".
Right click it and select "Properties".
Click on the Compatibility tab.
At the bottom of the Properties dialog, in the "Privilege Level" section, put a check mark in the box labeled "Run this program as an administrator"
Click the OK button.
I also chose this time to change the port number VNC works on (I have several computers on my network, so I have my router configured with a custom port for each one). After I did all these things, VNC started working fine.