VNC + VIRTUAL DESKTOP + JAVA ACCESS = WIN?

Ok, heres what I want to do:
I want users to be able to connect to my website and through some sort of client, use a sort of VNC on a "virtual computer" that doesnt actually exist. I'm pretty sure they can do this at Flinders University, so I know its not impossible.

If anyone can tell me how to do this, I shall reward thou with CP.

OP again.

OK, problem TWO, same scenario as before but it cannot run on an OS because there is no OS, that is, you cannot run VMware Server because it would require you installing it to a HD and then executing from inside an OS.