xVNC is a Windows application that combines a remote-installer and a VNC
system all in one. With it, you can remotely install VNC onto a target 
machine and immediately connect to it. xVNC is, by default, deinstalled when 
you disconnect. Access is restricted, of course, to domain administrators.
If you're looking for a quick, unobtrusive way to VNC into to a Windows 
computer that doesn't have VNC installed, this is it.

Windows-only, open-source. You can get a copy here:

http://xvnc.sourceforge.net/