Desktop Vs. Laptop
Portable computers already have a built-in monitor, it's one of the things that differentiate them from desktop tower computers. Laptop computers, without fail, have a port for an external display. Desktop computers are limited only by their motherboard and incidentally, the video card. Old computers, which still have the AGP port are more complicated to upgrade to dual monitors, in case the current video card does not have two ports.
If it does, your chances are looking terrific. Port broken? Desktop computers can be expanded via an additional video card, if the motherboard accepts PCI-E cards. If it does not, jump to the solution for laptop owners.
USB 2.0, fortunately, is one of the common ports, there are a couple on every computer. There are USB to VGA/DVI external video cards. Check what resolution they support, Full-HD is desirable, considering how wide-spread it is amongst modern LCD monitors. Having two monitors is simple. What if you wanted to have two extra monitors beyond what's in the laptop already? Connect one to the video connector, one to the USB external video card, and you're set to go. These USB devices can be hung from every USB port of your laptop, if you like, supporting up to 4-6 extra monitors. One solution on Windows XP and above is MaxiVista, Xinemara on Linux and ScreenRecycler on Mac OS X. Conclusion
Just like in any other case, where you're figuring out how to position monitors on your desk, pay attention to ergonomic and health safety guidelines. You usually don't have to take any further actions to set up your PC to run on dual-monitors. In the few rare cases you do, upgrading to a two video card setup, setting up a virtual monitor system with other computers, or USB video cards help you do it effectively.