You might be in the situation to have a desktop PC + a laptop.
If this is your case and you feel the need of having two monitors and you don’t want to buy a new one, I suggest a product: MaxiVista.
Super easy to deploy, super easy to use, great feeling from the application: it works with WinXP and Vista.
I’m trying the Free Trial and it’s really cool! The only concern I have is that the monitor attached to the desktop is by far greater than the monitor of my laptop and this is a bit confusing when moving the mouse from one side to the other.