Left click on the Print Scrn (Print Screen) key. 

Open Irfanview

Left click on Edit and select Paste

Left Click on File and select Save as... S 

Set the options as shown to reduce the size of the file and still maintain reasonable quality

Select a folder in the Save in: (recommend starting with My Pictures) field or Create New Folder

Give the New Folder a Name

Left click on the Open button to open the New Folder 

Give the picture a name in the File name: field

Save as type: JPG - JPEG Files

Left click on the Save button 

If your screen is merely dusty, you can wipe it gently with a soft, clean cotton cloth.

If your screen is downright filthy, you're going to need to resort to stronger tactics.

Do not use compressed air.

Do not use a glass cleaner like Windex or other ethyl alcohol/ammonia-based cleaners.

You can buy special cleaners, but this is sort of throwing money away, since most of these are "ionized" water. You can buy a liter of distilled water cheap and fill a clean spray bottle with it. Most distilled water is "ionized". If you feel that you need more than water, add a teaspoon or two of white vinegar to the water in the spray bottle and shake to mix the solution thoroughly.

Always use a soft, clean cotton cloth rather than a paper towel or some other type of cloth. Spray the water or water-vinegar lightly on the cloth and then wipe the screen until all the smudges, dirt, and other crud is completely removed. It's best to use a lot of light wipes with the cloth rather than pressing too hard with the cloth trying to remove everything quickly.

Do not touch or press on your LCD surface too hard or with your fingers, it can cause pixels to burn out.