One widely available software is MS Paint which is available among Accessories in all computer running the Windows OS. At the screen where you want to take a screenshot, press the Print Screen (PrntScn) key to copy image into Clipboard. Open Paint, then click the down arrow below Paste and select Paste (see screenshot below):
|MS Paint, Paste from Clipboard option|
Whatever was in the former screen at which you clicked PrintScreen will appear in Paint. Save and it is done if you want the whole screen. However, if you want only part of it, then you should crop.
To crop, click the down arrow at SELECT icon and select the shape you want - Rectangular or Free Form:
|MS Paint, Select Shape|
Then click the CROP icon
Save and it is done.
You can also easily do screenshots with the free Irfanview photo editor. Note that I like to use both PC Paint as well as Irfanview. This is because certain things which cannot be done with one can be done with the other, and vice versa.
For example, you can draw lines and shape with MS Paint to highlight certain part of the screenshot, but you can't do that with Irfanview. But if you want your screenshot to include the mouse cursor, it cannot be done with MS Paint, but it can be down with Irfanview.