Hi illy2! Thanks for using the Apple Support Communities!
I have found a couple of articles that will address your issues with iTunes video playback. The first is the article you referenced in your post:
I would like to point out this section of the article to you:
High-bandwidth Digital Content Protection (HDCP) is a security feature requiring the use of HDCP certified products in order to display digital media like High-defintion (HD) video. In iTunes for Windows, in order to view HD content, both the display hardware (computer screen, LCD, external monitor, and so on) and video card driver must support HDCP.
Bearing that in mind, you may want to check this article to make sure your computer meets the necessary system requirements for HD video playback, assuming that HD videos are what you are purchasing/downloading:
If those requirements are met, then you might want to refer to this article for Windows 7 or Windows Vista:
or this article if you are using Windows XP: