Moving old HDD to new PC with Windows

I have an old computer I wish to move the HDD from to a new computer that is already running Windows 10 on it.

I'd like to be able to save all the data on the old HDD and use the Windows 10 installation that already exists on the new computer rather than the old HDD.

My question is, how do I safely remove the Windows 10 install from my old HDD without harming my files so that there is no conflict/dual boot on the new PC that already has Windows 10?

When two operating systems are installed on single hard disk drive its refer to as dual-booting. You can connect the old HDD as a secondary drive and manually delete Windows OS.

The system won't try to boot with the old OS files?
Also what directories/files should I be looking for to ensure the old OS files are removed completely?

If you change BIOS boot order to your old HDD BIOS will try to boot Windows from it.

You can delete any directory you want you might have to take ownership of the files/folders before you can delete them.

Make sure you boot from the new HDD with the old one connected to a SATA port.
The best thing to do is to make a disk image and save it to your new disk or to an external disk so you can recover it if anything goes wrong.
All your data should be under X:\Users\NAME where X is the drive letter of your old disk.

I would copy the data from the old disk to the new one and reinstall the old disk back on the old computer to make it functional.

Megahertz said:

Make sure you boot from the new HDD with the old one connected to a SATA port.
The best thing to do is to make a disk image and save it to your new disk or to an external disk so you can recover it if anything goes wrong.
All your data should be under X:\Users\NAME where X is the drive letter of your old disk.

I would copy the data from the old disk to the new one and reinstall the old disk back on the old computer to make it functional.

The old PC is so old and dysfunctional to begin with, which is part of the reason for transferring files to a new PC setup.
The plan is to get rid of the older PC as it is a well outdated, pre-built that is likely over a decade old now.

I am planning to do this HDD move for a family member, who often is growling at their older machine for it's slow, delayed and just generally super sluggish behavior.
I am also frustrated with trying to fix it's many problems over the past 5 years. Haha!
So keeping the old PC functional isn't a plan in any way.