Welcome to the TH Community, @Ominator!
Congrats on your first PC rig!

However, the new motherboard on the PC build requires you to install a new genuine Windows OS. You will most definitely be able to use the old hard drive, once you re-format it. Keep in mind that this would erase all its contents, so you need to backup the data from it somewhere else first.
The operating system communicates with the motherboard, the new PC doesn't recognize the system because it doesn't belong to that motherboard. You won't be able to use the same OS activation code/product key either, unless the new PC's motherboard is identical to the old one. For more details, contact
MS Support.
Once you have the new OS installation disk and you have backed up the data and reformatted the HDD, you should be able to start fresh and use it for the new Windows OS and your data (from the back up drive). Yet again, you will need to reinstall all the software/programs as well, because you won't be able to launch them on he new system.
Good luck! Let me know if you have more questions!
SuperSoph_WD