Recently I installed a whole bunch of video editing utilities including Nimo's codec pack. Now my media play wont play back certain video files, including most xvid files and MS MPEG4 V3. My computer would just sitting there displaying a blank screen on media player, and the hard drive light would be constantly on.
I have reinstalled a couple version of Nic's xvid codec, but still no playback.
I've never had much luck installing codec packs, there's too much to go wrong. Especially with those packs which try to "help" you, by changing the delicate balances which previously let everything work fine...
My suggestion is to install codecs as and when you need them. Windows Media Player is useful if you want to automatically download and install codecs, but it doesn't find them all; I tend to download individual codecs, and keep them in a folder for when I do a new Windows install.
One thing that might help is reinstalling a DVD player. Some files use odd combinations of codecs including elements from a DVD codec.