If you have access to the MySQL error log check in there to see if there are any issues. Also with the user you have created the event with try executing the UPDATE statement manually to see if this causes any issues. As the EVENT runs in the background any permission problems on performing the update will only be logged to the error log and not visible on screen.
Did you check the MySQL log file to see if there are any errors? I am not familiar with Windows installation of mysql but if you check the my.cnf which defines the parameters of the database server you should find something like err-log=/var/lib/mysql/mysqld.log
which indicates the location of the error log. I am sure something will be posted in there as to why this is not working.