SQL SERVER 2005- Error 18456: login failed for user

SQL SERVER 2005- Error 18456: login failed for user ' Computer name\ UserName'
This error happens when I run Microsoft SQL Server 2005 at first time with windows vista; I login using SQL Server authentication that was successed, but when I tried to login using windows authentication that was failed following by error message



To fix this problem
Following next steps:
1. Login using SQL Server authentication.
2. Security>> Logins >> New Login



3. In General
Login name= Computer name\ UserName
Select windows authentication



4. In Server Roles
Select sysadmin
>> ok



Now, login using windows authentication that will login.