it sounds like a windows password, but why it is asking you for it there, i am unsure. It could be a conflict with your drivers of your old mobo and when it tries to use them your new one doesn't recognize the commands. 
I have seen some odd results from a mobo switch, but there are some things you can try. 
Can you boot into safemode? if so try uninstalling all the old mobo drivers and reboot. 
*you may end up putting the old mobo in.. but that is a lot of work, so i will try and not make you do that, unless it is the only option.   
Try booting to a floppy as well (command.com) and try accessing files on the drive, this ensures that your mobo can read your hard drive.  Which if possible likely means a mobo switch or a Windows re-install.    
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