After you launched your Windows instance you can either log in through the instance console or connect using rdp but then you need to enable a security group that allows tcp port 3389 and a floating IP.
A user called “Admin” is created during the instance initialization and you can retrieve the password with the following command.
nova get-password <instance id or name> <path to SSH private key>
You can also log in using the Windows “Administrator” user. You can set the password by logging in with user "Administrator" on the Instance console and you will be prompted to set the password.