Changing Windows 7 default user on bootup

it is ultimately controlled by registry but you dont need to go this far, there's a nice handy tick box you can change (and then provide default logon credentials)

Press windows & R for run box

type "netplwiz" or "control userpasswords2" - either will work fine

Untick "Users must enter a user name and password to use this computer."

You will then be prompted to enter default login details, just OK this box unless you have a password in which case type this in, then apply and restart, PC will Auto-login.

I found that it was this:

HKLM\Software\Microsoft\Windows\CurrentVersion\Authentication\LogonUI

LastLoggedOnSAMUser = Computername\username (or domainname\username for a domain account)

LastLoggedOnUser = .\username (or domainname\username for a domain account)

This worked for me, went to
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon
and changed DefaultDomainName and DefaultUserName values to the desired string data.
Additionally for auto-login it needs to be set (create it if it does not exist) the DefaultPassword value with the user password as string and also the string value AutoAdminLogon should be set to 1.