Try VLC PLAYER next.
Also, the full error message and which 7?
I bought some new dvd's recently, and WMP fails to play them. The system says my device is not signed.....but it works fine some of the time
Should i maybe remove, then reinstall my DVD/RW?
Dell 0J584C
Win7 OS
Samsung DVD/RW

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic