RealVnc does all what you want. It can be set up in server-less mode, and it can be installed even as a windows service, making it running even on a login screen.
Easy setup: well, that is relative. It has a GUI installer where you can click everything, but you need to understand which option exactly what means. Check also the firewall settings, possibly you need to allow the incoming tcp port 5900 manually.
It can be password-less (insecure), use a pre-defined password and possibly also other authentication methods exist.
RealVnc istself is not opensource, but the VNC protocol is; which results that numerous client and server versions of the protocol are also available (in Linux environment, I use x11vnc for the server and xtigervnc for the client side).
- it lacks sound support (you can transmit also the sound channels with pulseaudio)
- not all client work with all servers, try before productive usage
- client-side bitmap caching is sometimes buggy, resulting slow scrolling. This can be solved by a correctly chosen, and correctly configured client/server side.