3 monitors on a Windows XP machine

http://www.microsoft.com/windowsxp/using/setup/learnmore/northrup_multimon.mspx
http://daggle.com/060223-231233.html

Excerpts from the above links -

What is MaxiVista?
MaxiVista turns any networked computer into an external monitor. MaxiVista's only hardware requirement is that the two computers must connect to each other across a network, so it's perfect if you can't add an extra video adapter. I have a network adapter for my old laptop, so I connected it to my network and installed the free MaxiVista demo. It worked and my laptop's desktop was immediately spread across three displays, as shown in Figure 1.

Manage Multiple Monitors with Ultramon
There's a downside to using multiple displays—it's hard to keep track of your windows. Arranging windows on each of the different displays requires dragging and dropping, which takes as long as switching between windows with the taskbar. The Windows XP General newsgroup has many questions from people with similar complaints, and everyone gets the same answer: Ultramon.

Ultramon makes it easier to work with multiple displays by adding a taskbar to each monitor, adding a button to the title bar of every window that you can use to move it to a different monitor, and letting you quickly switch between different monitor configurations. I configured Ultramon so that pressing F4 moves a window one display to the right and pressing F3 moves it one display to the left, as shown in Figure 2.