Does standalone MySQL 5.1 installer succeed in installing?
See: https://forum.team-mediaportal.com/.../1-1-0-alpha-fails-install-mysql-point-67435/
In short: after uninstallation of MySQL the database was not deleted and so the installer cannot set a new password.
Solution: Uninnstall MP and MySQL, remove all MySQL data files (usually C:\Document and Settings\All Users\Application Data\MySQL), then reinstall MP.