SQL Express 2005 error

I'm connecting to my local SQL Express 2005 installation. In SQL Management Studio, I attempted to delete a database by right-clicking on it and pressing delete. I had the following checkbox ticked... 'Delete backup and restore history information for databases'. When I attempted to delete, it gave an error about it being unable to delete the backup. I unchecked the box, then it was able to successfully delete.

Now if I try to add a new database, using any name, I am getting an error:

Msg 262, Level 14, State 1, Line 1
CREATE DATABASE permission denied in database 'master'.

I have always logged in using windows authentication and in the past performed these actions numerous times without a problem. Somehow my permissions have changed.

I went into Security...Logins... BUILTIN\Users and noticed that this user only has the 'Public' server role. Is this my problem?

Also, I cannot recall the sa password. When I attempt to login using sa, I get a message:

Login failed for user 'sa'. Reason: The account is disabled. (Microsoft SQL Server, Error: 18470)

The data on this SQLExpress installation is only for test purposes, so if I lost data trying to fix this problem, it wouldn't really matter.