You can replace your HDD with a SSD which will greatly improve your system performance. For installation media, you can use the Windows 10 Media Creation Tool to create a Windows 10 Version 21H2 Bootable USB. The Windows 10 Activation Mechanism is tied to the motherboard, not to the Hard Drive. When you first upgrade, your system details (mainly motherboard) are submitted to a Microsoft Product Activation Server. When you reinstall, you can just select I don't have a Product Key and then select the correct Edition of Windows 10 in the dropdown list that displays in the next screen. Select Windows 10 Home if you had Windows 7 Starter, Home Basic or Premium or Windows 10 Pro if you had Windows 7 Professional or Ultimate. After installation, when the system connects to the internet, the system (mainly motherboard details) will be resubmitted to the Microsoft Product Activation Server and your Device will be activated. This happens automatically within the background.
I have an old Inspiron desktop and the hard drive went bad.  The computer came with Windows 7 pre-installed on it and it had been upgraded to Windows 10 some time back.  If I buy a new hard drive, will I be able to install Windows 10 directly using the Windows 7 product key I have?  Also, where would I be able to get the Windows 10 download as the computer didn't come with a Windows CD, or maybe I just can't find it. Thanks. John
Clean install of Windows 10 for new hard drive using Windows 7 product key