Sony Vegas Pro 8.0 video making help?!

I downloaded Sony Vegas Pro 8.0 a couple of days 以前 and began the tutorials to learn how to use it. The only problem is, I cannot actually view the video clips in the video 预览 when I add them to the timeline.

I can view the clips when I have them in the 'Trimmer', 'Explorer' and 'Project Media' section and they work fine. I've tried all sorts of file types and I still get the same problem. I don't have a clue what is going on here, does anyone have any advice?