I had similar problem while installing SQL Server 2005 on Windows 7 Professional and got error SQL server failed to start
. I logged in as a Administrator (my user id is administrator) in windows.
SOLUTION
Go to services, from control panel -> Administrative Tools
Click on properties of "SQL Server (MSSQLSERVER)"
Go to Log On Tab, Select "This Account"
Enter your windows login detail (administrator and password)
Start the service manually, it should work fine..
Hope this too helps..