The simplest one is:
1 Open microsoft paint.
2 press the print screen button on your keyboard
3 In microsoft paint press "ctrl + v" or go to edit and select and click paste:
This will capture an image of your entire desktop (screen) including video.
Once you have mastered this go to step 4.
4. Now you can open the video and pause it at the desired frame. then press "print screen" (whilst still in full screen mode for best results)
Paste in microsoft paint (ctrl + v or select and click):
If you are happy with the capture save it (giving it a new name including a number e.g "capture1" "capture2" etc.)
Hope this helps