I've got a linux box that is running dual-monitor with extended desktop from one to the other. I just use the simple vncserver setup (vncserver -geometry 1440x800) . When I then access it from my windows box with VNC Viewer Free Edition for Win32 and one screen, it opens the extended desktop screen?
How Can I change to the other desktop or just make it logon to the other screen?
Not easily. The VNC viewer will show the desktop as it is on the server, irrespective of how many monitors it's spread over. If you're running vncserver as a standalone desktop (i.e. not as a copy of your main display on the Linux computer) then it may be better to start it at a size that is convenient for the viewer machine you intend to use.
On my server, I run a vnc session that I connect to using ssh over the internet, and I set the desktop res on that screen to be the same as my laptop for convenience - i.e. at work it shows nicely in a window, and works beautifully fullscreen on the laptop.