WMP and WMM codec help?

I record for YouTube using both CamStudio and Fraps, but recently all AVIs I record appear black in Media Player (can hear the audio) and the clips have yellow caution signs in Movie Maker. This is a big problem, because I use Movie Maker to render and post my clips. I can still see thumbnails of the videos in their folders, and they'll play in the CamStudio 2 player. I think I may have deleted a factory installed codec accidentally when trying to free up some space. What codecs are necessary to view screen captures?