Greetings,
Welcome to the forum.
I am not a HP employee.
Is your current HDD still working or is it toasted?
Did you buy a new HDD? I suggest you buy a SDD. SSDs are more expensive but the increase in performance is worth the cost.
Your PC shipped with Windows 8.1 but you have updated to Windows 10 Home 64 bit (W10). This means your PC has a digital entitlement for Windows 10 Home, which allows you to install W10 clean on a new storage device such as a HDD or a SSD.
The new W10 install should activate when you are connected to the internet.
You can clone the current HDD to the new HDD or SSD if your current HDD is still working (Link).
You will have to go here (Link) to download Microsoft's W10 Media installation Tool on a different PC if you must install W10 clean.
Run the Tool. Select "Create media for a different PC". Select W10 64 bit.
Create bootable W10 USB media.
Install the new drive in your Slimline PC.
Boot to the W10 USB installation media.
Select "Custom Install".
Select "I don't have a product key".
Follow prompts to install W10 on the new drive.
W10 should install required drivers for your PC. Go here (Link) to download any HP software you may need to get your PC as close as possible to the HP factory image.
Some software provided by HP in the factory image may not be provided at the last link.
The last option is to order Windows 8.1 64 bit HP Recovery Media at this site (Link).
You can install the HP factory image and then upgrade to W10.
Regards
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