If you installed a codecpack, I suggest to uninstall it and install only codecs you really need. I don't like these codec packs because they installs lot of useless (and often messy) things.
Go to control panel --> Add/remove Programs.
Search for the codec pack in the list, and press the "uninstall" button. Then, install only Xvid codec (this is sufficient, and also free, to convert DVD into avi).