New MoBo and CPU, Keep old files?

Your not changing the hard drive so you can use the same hdd and just leave everything there. Just install all the new drivers and get rid of the old ones.
The other thing that you can do is that you can make a small partition on the hdd and put your files there and then format the other part and reinstall windows and all the new drivers for the new motherboard.

For such a significant change in hardware, a Repair Install of the OS already on your old HDD to fix driver inconsistencies could fail. You say, in fact, you don't really care whether you save the OS or get a new one.

The suggestion to make a new second Partition on the HDD and copy all old files there, then Format the old Partition to Install a new OS on, has a problem. I expect that your old HDD has ALL of its space allocated to the only Partition on it, and that is your C: drive. Windows generally will NOT allow you to change the size of the C: drive containing your OS, so you will not be able to shrink it to create empty space for the new Partition. (Such a process can be done, however, with third-party utilities, as long as your current old drive has enough empty space on to allow this.)

The simplest path is easy, but costs a bit of $. Buy a new HDD. Install ONLY that new drive in your rebuilt machine, then install a new OS on that HDD. When it's all running, you can shut down and install your old drive as a second unit. Assuming your old HDD is a SATA drive, do NOT get hung up on "Slave" drives and jumper settings - there are NO jumpers to adjust on SATA drives! You just plug it into a free SATA port, add its power supply connector, and go.

The complication here, however, is that all the application software you already have - like Microsoft Office, graphics software, browsers, etc. - will NOT be transferred this way. These days the original installation process for application software makes important entries into the Windows Registry. If you install a new OS on a new HDD, its Registry will know nothing about any software on your old drive. So you actually have to re-Install each application software package under the new OS.

Once you have done that, I suggest you copy all your old data files from the old drive to the new one, creating apporpriate folders for them as you go. After you've done that and waited a while to be SURE you copied everything, you could just wipe your old HDD clean and use it like a brand new empty HDD for data storage only.