First you need to stop all the services for each instance. I wrote here how to restart them. Just stop them instead of restarting them.
Right click the service and click Properties
Click the Advanced Tab
Locate the properties box
Scroll down to Startup parameters
Move to the end of the text that is already entered. With no spaces enter ;-m
Click Ok and start the service
This starts the service in single-user mode.
With a domain admin account, launch a command window and type sqlcmd and press enter.
You will see a > Type the following:
EXEC sp_addsrvrolemember 'domain\user', 'sysadmin'; Hit enter.NOTE: Domain would your domain IE: abc.com and user would be jsmith.
GO Hit Enter
Type Exit, Hit Enter
Open SQL Server Management Studio and use Windows Authentication
At this point you should be able to now login with that user account.
Go back and stop the service, remove the ;-m and start the service again.
This was successfully tested in SQL Server 2008