virtualbox.org

how can I auto-fill in user name and secret when start up windows 7?

If you want it to log in automatically on boot, why password protect the account at all?

You can send keystrokes into a VM using VBoxManage, the trick would be knowing what Win7's current state is. I don't know of any elegant solutions (meaning solutions that I can be sure will always work).

Passwords on an account are necessary if one wishes to easily access shared files & folders, which sometimes get harder if there's no password for the account.
Meanwhile, auto-log-in is possible in the guest OS without having to do anything with Virtualbox. Google "windows 7 auto log in"