Hi, open the video with media player in full screen, press pause on the frame you like, press print screen (my laptop have button "Prnt Scrn") , then open Paint (windows paint) , paste screen shoot and save as JPG, can crop unwanted frames or edit with photoshop for better quality edit: contrast, sharpnes, colour
or if you have video editing program like sony vegas, also can take a screen shoots (still) with the program
cheers