Generally if you get a new motherboard it is a good idea to fresh install
is there anything on C drive you want to rescue? try making this on another PC: http://www.howtogeek.com/howto/windows-vista/use-ubuntu-live-cd-to-backup-files-from-your-dead-windows-computer/
change boot order in BIOS so USB is first, hdd second
boot from installer
follow this guide:
http://www.tenforums.com/tutorials/1950-windows-10-clean-install.html
when you reach the screen asking for licence, click "I don't have a key" and win 10 will continue to install and may reactivate once finished**
On the screen where you choose where to install win 10, if it gives you an error about GPT drives, delete all the partitions on the hdd and press next. If it still gives error, cancel out of the installer and restart PC and start installer again, it will accept next on that screen this time (some PC just need a restart here)
** I say may reactivate as it depends on if your licence is attached to your current email address or not. What you should have done before swapping motherboards is follow these steps -
Reactivating windows after hardware change
the 2nd step there might work now if you using same email address. Otherwise you may need to go to settings/update & security/activation and contact MIcrosoft - should be a number at bottom to call them on, and explain what you did. Helps if you have your OEM key, they should swap activation to new motherboard.