Download a fresh copy of Avast and reinstall

Basically it is just a check that you can access those areas of the system as the malware will try to kill them

Could you check this page for possible resolution to the media player problem http://answers.microsoft.com/en-us/windows/forum/windows_7-sound/windows-7-media-player-server-execution-failed/ecc08c3b-d445-48c2-b07d-9df48500434f