Okay. I tried un-installing again and reinstalling with only USB and network (the features I actually need) and it still hangs at exactly the same point for me (with VBoxSVC maxing-out one of my cores). I was able to narrow-down my own particular problem to the USB component, as when it is not installed VirtualBox boots normally.
I started a ticket for my own particular problem here: http://www.virtualbox.org/ticket/6538
. If you believve you are having the same problem (specific to Windows 7 x64 and the USB component causing virtualbox to hang before the GUI loads) it may be useful for you to add your own experiences. Otherwise, you may be better to create your own ticket.