This is straight from Windows Vista help. Search help for 'logon' for more tips.
You can use automatic logon to log on to your computer without having to type a user name or password.
Warning
Using automatic logon can pose a security risk because anyone that has access to your computer will have access to your programs and personal files.
In the Search box, type control userpasswords2, and then press ENTER. If you are prompted for an administrator password or confirmation, type the password or provide confirmation.
Clear the Users must enter a user name and password to use this computer check box, and then click Apply.
In the Automatically Log On dialog box, type your password, and then click OK.
In the User Accounts dialog box, click OK.
After you restart your computer, you will be able to log on automatically.
