When installing SQL Server 2005 SP3 you may get an error about the password being to short, and it may have nothing to do with your sa password. It’s because the Install Scripts are trying to create a certificate with a password which doesn’t meet your domain’s password requirements.
If you get this error navigate to the C:\Program Files\Microsoft SQL Server\MSSQL.1\MSSQL\Upgrade folder. Find the script for upgrading the SQL Agent. Open the script and find the CREATE CERTIFICATE statement. Make the password longer. In my case I added 123456@ to the end of the password. The password is in two different places in the script.
Then open Add/Remove Programs and select SQL Server 2005 and select Change. Then run though the wizard to complete the installation of the service pack.