A codec is simply a file used by your operating system to translate the code in a video file, into something your media player can read. If you just type xvid into Google then you should find all the info/files you need. Alternatively you could download VLC which is a media player that has most common codecs built into it. However, I run it on a Mac, and have heard VLC isn't as user friendly on a PC.
Hope that helps man.

EDIT: Tubes, I like how we both responded with almost identical responses at exactly the same time. Cheers Mate