Missing Codec

[Ken]

Hey There:

Ok, I have windows XP and have been able to download
videos off line and watch until today.

Today I downloaded a few .avi file videos and when I play
them on media player...I get error codec message.

Question, do I have to download a special codec file or
was the video that I download corrupt?

Any help as usual will be greatly appreciated.

Regards,

Ken