The codecs are the plugin, allowing programs that aren't specialized for video to show video.
Anyway, I don't know what to tell you about your install problem. I downloaded and installed
1105 Standard and the
1106 update from
Codec Guide (using MajorGeek mirrors; these were current as of this post) and everything worked fine. You could try popping in on their forum and asking for help.
The
CCCP is another codec pack you could try.
Edit: Heh, I got distracted testing different encoding formats in Windows Media Player (a program I don't use that often) and David posted before I finished this. :)