I've got a Sony Vaio VPCEB47GM. It also has a black screen flash real quick at the Welcome page. I'm thinking it might be the Windows Boot Manager. In a dual boot set up you usually get a black screen like this:
Then you would select which operating system you want to boot into. Since we both apparently have just one operating system installed, I'm thinking this is just Sony's way of bypassing the Windows Boot Manager screen and going to the Welcome screen. And I'm not sure that there's any way to change the way Sony boots into Windows 7.