Comment 6 for bug 530481

@Reinhard
if you don't have an Nvidia card you should not install the nvidia driver (unless you know what you're doing). In this case the driver was pulled as a dependency by mplayer but this should not happen any more after you fix mplayer (as vdpau is a separate library now).

For example if you want to keep the Nvidia driver installed (e.g. if you plan on adding an Nvidia card) and keep using open drivers you will have to do the following after installing the nvidia driver:
1) sudo update-alternatives --config gl_conf (and select the alternative provided by mesa)
2) sudo ldconfig
3) sudo update-initramfs -u

Usually proprietary drivers are installed through Jockey, which hides these details from users.

For further information you can have a look at the wiki:
https://wiki.ubuntu.com/X/Troubleshooting/NvidiaDriverSwitching