Why isn’t iTunes video playing on Vista?

iTunes is a resource heavy software, it requires a lot of processing power to run videos. The first thing you should do is go to iTunes download page and check if your computer meets the system requirements mentioned there. Next check if you have updated graphic and video drivers, you can update them via the Windows update. Though windows update usually updates old drives, it might not be able to update some special hardware drives. To update them go to device manager from Control panel, choose the device and click on properties to get the actual name of the device and manufacturer. Go to device manufacturer’s website and download the driver for your hardware directly from there. Make sure you are downloading the correct driver for your device; a wrong driver is capable of corrupting your device. You can also try to increase the display colour quality of your device. Go to display settings and select the resolution that Windows recommends. Make sure you are doing this after you update your display drivers. You should also disable Direct3D acceleration in QuickTime which is responsible for video playback in iTunes. To do that, go to QuickTime preference and select the Advanced tab and uncheck Enable Direct3D video acceleration under Video category.

Something much simpler would be to just use VLC Media player for all video playback instead of iTunes and QuickTime.