I didn't know the existing sa password so this is what I did:
Open Services in Control Panel
Find the "SQL Server (SQLEXPRESS)" entry and select properties
Stop the service
Enter "-m" at the beginning of the "Start parameters" fields. If there are other parameters there already add a semi-colon after -m;
Start the service
Open a Command Prompt
Enter the command:
osql -S YourPcName\SQLEXPRESS -E
(change YourPcName to whatever your PC is called).
alter login sa enable go sp_password NULL,'new_password','sa' go quit
Stop the "SQL Server (SQLEXPRESS)" service
Remove the "-m" from the Start parameters field
Start the service