1) it's a common problem, even for people who do a CLEAN INSTALL to new hardware
2) one quick fix is to do an "In Place W10 Upgrade" which basically is:
a) add an 8GB+ USB stick
b) download and run the MS Media Creation Tool
https://www.microsoft.com/en-ca/software-download/windows10
c) run "setup.exe" from it while still in Windows after its been created
d) let finish
3) CLEAN INSTALL and LICENSE:
If you upgraded to W10, then swapped the CPU/mobo then you have no license. You can also end up with various issues by swapping the mobo/cpu to an existing Windows install.
So you should always do a CLEAN INSTALL of Windows when you swap to a new motherboard (exceptions exist but the more different the mobo is the more likely problems will crop up).
Summary:
a) In-Place Upgrade is a quick and easy first step. May actually work as good as a clean install but hard to say.
b) License probably needs to be purchased to activate (forget what NOT activating causes now... just reminders?)
c) CLEAN INSTALL is recommended but plan carefully (games? Passwords? Windows login? e-mail?)
Just FYI, but Steam games can be moved fairly easily. For example, let's say you buy an SSD to put a clean install of Windows on (min 120GB). You could then make the folder "E:\STEAM" on the same HDD (if that's what you had already... a single hard drive) then cut/paste your entire "Steamapps" folder from its location at: C:\Program Files (x86)\Steam
I would make other folders too so you can move important files too including the entire "Documents" folder as most SAVE GAMES exist there so you can move them back as needed (if not on Steam cloud)... then DELETE all the Windows folders on the HDD.
Finally if you go with a new SSD/HDD for Windows I'd backup the C-drive with a tool like Acronis True Image once it's all configured correctly.