Is this a common problem with Avast? All other video programs work, just not windows media player. I've tried restarting the service and that does not work.
No, not common at all.
Are you sure it is actually executing WMP, and not some other executable pretending to be WMP?
Either way I would highly recommend re-installing WMP.
If you are using Windows 7, it is done as follows:
Control Panel -> Uninstall a program -> Turn Windows features on or off -> Media Features -> Untick the Windows Media player checkbox, click OK.
It will then remove WMP from your system. After doing that, re-tick the checkbox and wait for it to re-install.
Hopefully this should resolve your issue.