Tight VNC

Tight VNC works on my windows xp machine unless I switch users on the server machine. I get an error that indicates another instance of Tight VNC is running.

It behaves this way whether I use server or service, and will only work if I log off the inactive user account.

Is there a way to use user account switching and VNC at the same time, or do I have to always log on one user at a time?