Well the sure fire method is to backup the data you do want to keep to a external storage drive via drag and drop copying, then disconnect this drive.
Next go to System Preferences > Accounts and create another admin user account, reboot and log into this new account and delete the old one, reboot and recreate that old user account again with the same name, then reboot and log into it.
Transfer files from backup into the new account with the old user name like before.
Another method is to use the free EasyFind and search for the extension(s) with a wildcard like so
*.mpg or *.mp4, or *.wma
when it finds them all in your account, then you can delete them right there in EasyFind.
If your doing this because your going to give up ownership, you might want to erase and install and leave just OS X liek a new machine