Press PrintScrn (Print Screen) on your keyboard, then open your image editor and
use the "paste" command ( Ctrl-V) to paste the screen capture into the image
editor. Resize as you see fit, jazz it up with a border or drop shadow etc etc
then save as jpg or whatever.
Import the picture into your web, insert the pic in the page.
Be aware that this makes a 'screen size' capture of the screen you are on when
you press PrintScrn and if you reduce it, it will only make it illegible if you
intend it to be read.
What is this for - samples of your web design portfolio or similar - like you see
around on other design sites?
eg -
http://members.iinet.net.au/~admurray/cgi-bin/skills/csvread.pl