Just solved a problem that hit my machine on Dec. 1st. Turns out there may be many people in the same boat.
Apparently there are video CODECS that "expired" on December 1st (happened last year to many people too, I eventually found out) and this year. I couldn't play DVDs at all in any program. I thought it was my MDAC or a virus but it wasn't and I came to find out the techs didn't even know about it until recently.
This adversely affected a student film project I was scoring.
I thought I'd post this as a "heads up" for what it's worth. The way it was repaired was to uninstall windows media player 11 (actually a "roll-back" to WMP 10) then re-install it. Then I uninstalled and re-installed my NERO audio/video burning software (the tech said ROXIO or other audio/video software would work as well) and all is well.
I've seen nothing about this on any boards until recently (on a Dell forum) and thought I'd share it.
In theory there is no difference between theory and practice, but in practice there is.