Manipulate SharePoint user login window

Hello.

Is there any way to manipulate the SharePoint popup login window or change it? What I want to do is to set an user name in the 'User name' text box through, perhaps, query string (I mean, having a link like http://site/subsite/main.aspx?username=myName and set 'myName' in the user name text box) or something like that. Is it possible?

Thanks anyway!!