Ctrl-Alt-Del to a PC not sending. Alt-TAB reveals this though...

I am trying to connect to PCs running UltraVNC servers at work using Apple Remote Desktop 3.6.1 on OS X 10.8.2.

But my attempts to send Ctrl-Alt-Del are having no effect. If I Alt-TAB it reveals this message.

I am assuming 'cad' is an abbreviation for Ctrl-Alt-Del. But say yes or no to this dialog makes no difference.

Has anyone seen this before and know what the solution is?

Thanks.