Many websites including our own use screenshots. To this you need a simple proprietary paint package
If you want to save the whole screen, just hit your Printscreen key. Now open your paint package and click edit -> paste and it should appear ready for editing.
To save just an active window click Alt + Printscreen at the same time (like this one below)
You can save you images as described elsewhere if you don't have a special graphics package
Please be wary of copyright when displaying images. Microsoft for example allow you to use certain screen shots provided you don't alter the appearance of toolbars