I had installed the Intervideo WinDVD demo stated above. After the trial period, the decoders (codecs) were disabled as well. I tried PowerDVD but it didn't work in WME only WMP. I was able to find a MPEG codec compatible with WME. It doesn't have a trial period, it's free, and it's only 100k zipped. You can find alot more free codecs here as well.
Free-Codecs.com