Google is your friend:
Way 1: Turning down hardware acceleration
1.Go to the Start Menu and select Settings->Control Panel.
2.From the list view, select Display Properties and select the last page titled Settings.
3.Find the Advanced button and then choose Troubleshooting.
4.There you will find a slider control for hardware acceleration. Move that slider to None, then click OK button.
This should allow you to use print screen, but some fast forwarding options might not function correctly.
Way 2: BSPlayer
BSPlayer is THE best movie player hands down. It works better with codecs than WMP or Winamp, supports a variety of file types, and has many extra features. I really suggest you use this player even if you don't need it for taking screenshots.
To take a screenshot, all you have to do is press "p". This will put the screenshot in your BSPlayer folder.
Way 3: Fraps
Fraps collects your FPS, takes screenshots, and records video of your game.The screenshot feature works well with movies. Just press F10 to take a screenshot, and it will end up in your fraps directory.