Understand that for licensing purposes, when "upgrading" to a new motherboard, that is considered a new computer and therefore, you need a new license for Windows, and perhaps other programs you have installed on your current computer too. The ONLY exception is if your current Windows license is a "Full Retail" license - that is, bought separately at a retail outlet AND not an OEM or upgrade license. An OEM (or System Builder) license is tied to the "original" hardware it was purchased for, or came with. Upgrade licenses are tied to the original license - typically an OEM.
Therefore, if your current Windows license came with your current computer, it is most likely an OEM version and cannot be transferred legally to this new computer. And so you would need to buy a new license, or use one of the free Linux alternatives.
But to answer your question specifically, since you are using a new motherboard (which consists of MANY distinct hardware devices on one board), new RAM, new CPU, and new graphics card, I would urge a new format and fresh install regardless. Remember, when any operating system is installed, it is configured for the hardware it expects to see with the hardware-specific drivers already loaded and configured in that installation. And simply slapping in an old harddrive with an OS configured for totally different hardware is likely to cause the system to choke upon first boot. A fresh install ensures proper configuration for all the new hardware.
So I recommend you copy off any data files you don't want to lose, then, with your new copy of Windows, format the drive and do a fresh install. That assumes your old harddrive is a nice SATA 3.0 drive - otherwise, if it is old technology, it will just be a bottleneck for your new hardware.