If you are having issues with video playback in iTunes for Windows, you may find the following article helpful:
I bookmarked this discussion, because I'm having the exact same issue!
Music videos play for a few seconds. Then the video suddenly almost stops, moving at a snail's pace with no audio (very slow counter). Pausing and resuming the music video only helps for 3 secs then back to slow mo.
iTunes on a HP Pavilion g4-110dx Notebook PC with Windows 7 Premiun Home edition.
I tried everything else in the suggested article to no avail. I'll update my discussion with my results.