Screen capture problems

hey guys, i need to take a screen shot or something like that of a Video File, its a .VOB file from a DVD, and i tried opening it with RealPlayer, Quicktime, Windowds Media, WinAmp, BS Player… but when i do the print screen thing, it is just a black square where the video would be, and i also tried 2 programs called Easy Screen Capture, and Capture Wizard or something like that, but it still doesn’t work…
you guys have any idea?