"Cannot open user default database. Login failed." after installing SQL Server Management Studio Express

This may not be answering your question specifically, but it will help other with similar issue

The problem is your user is defaulted to a database which is not accessible for any reason (can be renamed, removed, corrupted or ...) To solve the issue just follow the following instruction

1. Try to login again on the login page there is other tabs select "Connection Properties".
2. under the tab locate "Connect to database" and select an existing database you have access to like tempdb or master

3. Once you are connected to the SQL Server Instance execute the below TSQL to assign the login a new default database.

   Use master
   GO
   
   ALTER LOGIN [yourloginname] WITH DEFAULT_DATABASE = TempDB
   GO
   

Alternatively once you connected change your default database name to master via UI

Article taken from : http://www.mytechmantra.com/LearnSQLServer/Fix-cannot-open-user-default-database-Login-failed-Login-failed-for-user-SQL-Server-Error/

First, try to isolate your problem:

1. Take a backup of the file! Some of the steps below can, apparently, in some circumstances cause the file to vanish.
2. Are you sure you are connecting to the same instance through Management Studio as the program is?
3. If possible, try to shut down the instance that you are not expecting to use.
4. Set the user's default database to master and try to make the program logon.
5. Try to login as the user through Management Studio - since you have integrated security, you should open Management Studio as the program's user.
6. Are you using "User instances" - perhaps without knowing it? If so, this may be helpful: http://blogs.msdn.com/b/sqlexpress/archive/2006/11/22/connecting-to-sql-express-user-instances-in-management-studio.aspx

I haven't worked much with files being attached in the way your program does - but you write that you attached the DB in the Management Studio as well. Have you tried detaching it there before running your program? Perhaps you are seeing the Management Studio and your program competing for exclusive access to the MDF-file?

EDIT: I added point 6 above - this is new in my own list of TODOs when troubleshooting this type of Login failed. But it does sound a lot like what you're experiencing.

EDIT2: In the first edit, new item was added to the list. So the numbers in the comments doesn't correspond with the numbers in the answer.

I finally figured this out, and my situation is different than every other I've read about tonight.

I had restored my database from a backup. I knew that there was a particular login user that I had been using, so I created that user in SSMS. However, there was already a user by that name under the database that had come in with the backup.

Since I had screwed around so much trying to fix this, I wasn't able to delete the user under the DB easily. I deleted the database and restored again. Then:

1. Delete the user under the Databases->[my database]->Users
2. Create the user again in Security->Logins (not under your DB, although that probably works too.
3. Go to the newly created user. Select properties. Then under User Mappings, tell it to make your database the default. Give it read and write access.

Summary: I had two users. One that came with the DB, and one that I had created. Remove the one that came with the DB and create your own.