There are a lot of users experiencing this problem, I just don't believe there are enough to warrant an investigation into the matter. I posted here and on other sites about this issue; you wake up one day to use your computer as normal and no more WMP11.
There is a temporary fix if you want to watch videos and don't mind using Explorer. Find a picture to copy to the folder with your video files and then preview the picture, click the 'next' button to 'view' the next picture. Any movies that are in the folder will also be previewed.
Unfortunately this doesn't work for audio so there'll be no steaming radio or simple audio playing without using another program or another computer.
For radio, if you know the steaming URL, you can use Raima Radio to get your steams. There are other players you can use as well. Right now I'm using an MP3 player and speakers.
I tack this one up to the bad part of being a tech-innovator. I'm tempted to say it might be due to all the Ultimate extras that I've downloaded over the last year...you know...the ones that give added-value to Ultimate and...
Well, know that you're not alone on this.
Good luck.