Hello Mendeezee,
Depending on the age of the monitor, you may not be able to playback any HD content that is purchased in iTunes. One way to check is to download the SD version that comes with the HD version. Check out the articles below for more clarification.
iTunes for Windows: Unable to view or play HD video content that requires HDCP
http://support.apple.com/kb/ts3354
iTunes: Purchasing and viewing HD videos
http://support.apple.com/kb/HT3209
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.
Note: Depending on your computer's hardware limitations, you may be unable to purchase, rent, or view HD content from the iTunes Store. If you are trying to view HD video on an external display, the display must have a digital connection (DVI, DisplayPort, or HDMI) and support HDCP.
Regards,
-Norm G.