When setting the Run As property to SQL Authentication, you may get the following error message when you click the User Name text box: “Object reference not set to an instance of an object”.
This is caused by a bug in .NET Framework 3.5 (SP1), but is reported to be fixed in .NET 4.0.
To work around the issue, you need to disable “Advanced Text Services” found in Control Panel –> Regional and Language Options –> Languages Tab –> Details… Button –> Advanced Tab –> Check “Turn off advanced text services”