Okay, It sounds like that PowerDVD install is pooched up and that's where your machine is looking for it's DVD codecs. Can you do a full uninstall and reinstall of PowerDVD? (Do you have the program on CD or is it pre-packed with your entire computer's restore disc/partition?)
If you can't reinstall POwerDVD, I would give the codec pack in that second link I posted a try. That way you should get a clean codec install and you should be able to play DVDs using WMP.