|That's normal for the Guest Mode in Windows 7. it's very restrictive in what it allows you to do (even more than in previous Windows versions), and does not allow playing of DVD movies. Playing DVD movies works only in Standard or Admin user accounts.|
If it's your own PC you shouldn't be using the Guest Mode anyway, you should have already created a "Standard" or "Admin" account for yourself, and a "Standard" account for your other family members if required.
The Guest Mode (Guest Account in previous versions) is hardly ever used - I know of nobody who bothers with it because it's so restrictive. It's good to know it's available, but as the PC's owner you'd be rather silly to be using it for yourself.