There's no need to login iinto Windows (or Ubuntu for that matter) to change the boot device. That's done in the BIOS setup, not in the OS. It depends on the motherboard how to access the BIOS setup. Common keys to press early in the boot are del or F2, but it can be others also. See the user manual or motherboard manual, that came with the PC or the motherboard, or look if it is shown in a message on the screen.
I was trying to run ophcrack from a notable usb on a Windows 7 desktop and it was not working. I had the brilliant idea of running it on my 8.1 laptop. Unfortunately all I got was a black screen. Now I cannot switch the boot device back to the hdd due to the fact that I actually have to log in to due that and when I turn on my laptop all I get is a blank screen. I have tried booting from a Ubuntu live flash drive, but when I do it gives me the option to try Ubuntu without installing or to install it. When I select one of these I still get the blank screen. Also my laptop will not recognize my windows 8 recovery partition. What do I do?