I had a very similar problem last night.
Firstly,have you uninstalled Nero or Roxio ?
If so,you may think about reinstalling and see if WMP works...When uninstalling nero,i lost a codec....This was a problem i had a while ago.
Last nights problem was with my power dvd.This is how i fixed the problem:
Start >My computer > Right click on The dvd/cd Drive >Properties > Autoplay >
You should have a box possibly saying play music files....Scroll through the list and select dvd movies.
Try this and post back.