I installed MySQL using mysql-installer-5.5.20.0.msi on Windows 7. From the cmd window, I run \"mysqlshow -u root -p\". It asked for a password, then return
I had the same error. I resolved in this way
Go to start- MySQL Installer-community and run again the installer as a re-configuration(you will be asked so).
Once asked if you want make MySQL instance to run as a Windows service, check the box.
In case would do not work, try unistalling and installing again, and check the box to run MySQL as Windows service.