Here are precise instructions for taking a screenshot: (please note the second part is for Photoshop users, but it's more or less the same in most graphics apps)
Set up your desktop to look the way you want it to look...............then....
1. press the button on your keyboard that says "Print Screen".
2. Open Photoshop.
3. press ctrl/N on your keyboard, then click o.k. or hit enter.
4. press ctrl/V on your keyboard. the image is now pasted in the new document.
5. go to .. file/edit/save a copy.
6. choose where you want to save the image and the format you want to save it as. (.jpg .bmp etc.)
7. That's it!
Not so difficult really. If you are saving as a .jpg, set the compression at about 10, that way you get a decent quality that's not overly large in size.
Hope this helps.