This is how I get 4 screens for a workhorse computer and optional control on my second laptop.
Workhorse laptop is running Windows XP and a low-cost laptop running Windows Vista Basic Home (This Vista laptop is basically useless except I need it for testing.)
I have two old monitors. So each laptop gets a secondary display. Via the windows display property, I extended my desktop to these secondary display.
Now, on my workhorse, I have two screens. On the vista, I also have two screens.
But I want to have 4 screens for my workhorse when I am not testing using the idling vista. If I can control the vista from my workhorse, then I don't need to move my hand to the vista's keyboard. (BTW, my workhorse has an external keyboard and mouse which make editing so much easier!)
I bought Mirror Pro from Maxivista.com. (I am upgrading from a previous version, so I am paying only half price. If you don't need desktop mirroring, you can opt for the Professional. I would not suggest you to go for the Standard version!)
Install the software on my workhorse. During installation, it will prompt you for generation of client viewer. I asked for two client viewers.
Copy both client viewers to the vista and install both. Configure one of the client viewer to link to the laptop's primary screen and the other to its secondary screen.
From my workhorse computer, activate extended screens of both Maxivista viewers. I can see the background of the vista changed to the workhorse background. Open the workhorse display property and move the screens to their correct positions.
Viola! I use 4 screens at the same time. Optionally, I can control the vista using Maxivista too! Sometimes, finding the cursor from these many screen estate can be difficult. I use a custom set of cursors (brighter and bigger!) to help with my failing eye-sight!
This software solution requires both computer connected to a network, which is my case anyway.
There is no additional hardware and less cable than using KVM switch.
This is a printScreen of my current desktop.