Windows used to make it impossible to do this, windows 10 however is much better at it and it often works. It is a risk though, even if it works it could end up being buggy and either way you may have activation issues if you have OEM windows which is tied to your old motherboard with the licence.
If you use an online login for your windows it should be tied to your MS account which can make transferring it easier by logging in to the same account on the new machine and using the activation troubleshooter.
A clean install is the safest and best overall option, and should be done with only a single drive connected before adding others. If you have steam or blizzard games they can be copied across or left on another hard drive and will save a lot of downloading.
For steam you just reinstall it then create a new library in the same location as your existing steam folder and when you "install" a game it will find all the local files and use them instead of downloading. For Blizzard you can usually just run the game from wherever and it will automatically reinstall the launcher and point it to the game folder - at least that wrks for WoW and diablo 3, haven't got any others myself.
One final comment - you don't have to activate windows to use it, it works just fine unactivated.