There seem to be a couple of issues with UltraVNC and Windows Vista.
I am currently setting up a Conquest Dicom Server on Windows Vista Business Edition.
I want to use UltraVNC to remotely manage the server.
The problem is that with MySQL I need to disable UAC (User Account Control) which then causes UltaVNC to stop functioning.
Also you can not send the Ctrl. + Alt. + Del. command to Windows Vista with UltraVNC.
I found this article which seems to have the solution.