Your problem more than likely comes from your "codec pack".
As you've now (basically) trashed your system, i recommend you format/reinstall Windows XP, WinDVD, and MediaCar.
Then, install FFDShow (or 3ivX) and AC3filter. I have not come across any video files (other than QT/Real) that won't play with just those two codecs installed.
And if you need Quicktime/Real, then install RealAlternative and Quicktime Alternative