Hey there, cheesebagels!
@Pooneil is right! You won't be able to use your old HDD with your old Windows OS and the programs/games installed there, unfortunately. The new PC build and the new motherboard there requires you to have a genuine Windows installation! If you need to back up any data from it, I'd suggest you to do it on an external. Once you have the installation media and the HDD plugged in your new rig, you can start the fresh install and let Windows format the HDD for you.
Any kind of software from the drive must also be re-installed, otherwise you'd most probably not be able to load it or even if you do (which is highly unlikely), you'd face a lot of performance issues.
A totally clean start is highly recommended if you...