You could try doing a System Restore to a date prior to you changing the file associations.
Click on Start
Right click on Computer
Click on Properties
Click on System Protection (look for it on the left side of the screen near the top)
Click on the System Restore button
Choose a restore point that is prior to the date you changed the file associations
Follow the prompts from there.
Find an mp4 (or mp3 or flv, etc, whatever you use) in Windows Explorer. Right click it. Select Open with, then Choose Default program. Pick the right program and check "Always use..."
You could try doing a System Restore to a date prior to you changing the file associations.
Click on Start
Right click on Computer
Click on Properties
Click on System Protection (look for it on the left side of the screen near the top)
Click on the System Restore button
Choose a restore point that is prior to the date you changed the file associations
Follow the prompts from there.