Vista - cannot play video files on media center

I have a few video files on my computer ending in .avi and some ending in
..mpg. I used to be able to watch them all on Windows Media Center and now
when I go to play the .avi files, it tells me that "one or more codecs
required to open this content could not be found" and the film will not play,
however, the .mpg file will play. If I try to play the films on Quick Play,
the sound will play on the .avi films but there will be no picture. I
defragmented my computer a few days ago, would that be the problem?? How can
I fix this?

Liv