Good news, Maina: Windows (especially Windows 7, which you said is one of your preferred operating systems) is very adept at running multiple monitors, and there are few specific hardware requirements. In other words, any system capable of running Windows 7 should have no problem driving a second monitor.
If you’re a desktop user, check your video card to see if it has a second VGA or DVI port. If so, just plug in your second monitor and you’re good to go. (See below for Windows configuration instructions.) If not, consider upgrading to a dual-port video card, which you can do for under $50.
As for laptops, virtually all models have a VGA, DVI, or HDMI port for connecting an external display. So, again, all you need to do is plug in your monitor and configure Windows.
PC World’s Patrick Miller explains all this in much greater detail in How to Set Up Multiple Monitors, and also provides complete instructions on adjusting Windows’ display settings. Check there if you need more info.
However, if your system lacks the ports you need for a second and/or third monitor, one option Patrick neglected to mention is a USB adapter. A quick search for “USB to DVI” will reveal numerous products that plug into a USB port to provide a standard DVI (i.e. monitor) input. That’s a very convenient solution, though not always the cheapest: I’ve seen prices ranging from $50 to $120.