Today I had to log in onto a SQL Server for which the password for 'sa'-user was not known to me. After some Googling around, I found this post 'Forgot the SA password in Sql Server 2005'.
In short, what you do is loging on into your Operating System with an account that has Administrive rights, restart SQL Server (Express) in Single user-mode and login to SQL Server express with that account. This way you can change the password for user 'sa' or enable the account when it was disabled.
This turned out to work pretty well. I hope the link above can be of use to someone else.