Problem importing some .avi files into sony vegas, NEEDS HELP!

Ok im an anime/movie editor and I use sony vegas for my editing, at the minute im downloading an anime series called bleach however any episode past 166 will not work in vegas, only the audio will get imported and the actual video does not appear when I put it into a timeline, stopping me from editing. Is there a codec I need or anything like that because its really frustrating, also if you have any solution could you please not be too technical because im not so technical myself haha. This is what it says....

Documents\Videos\Bleach\Bleach 226.avi could not be imported. The required codec was not downloaded automatically because the Download codecs automatically check box is not selected in the Options dialog box.

What codecs do the files past episode 225 use?
Follow the instructions in http://www.moviecodec.com/topics/61931p1.html to find out

AVI files don’t work in Vegas on my PC either, only with me it lets me import the video, but when I drag it onto the timeline only the audio track appears, not the video track.
By the way, I edit anime music videos too. I’d like to see some of your videos!