Building a new PC. Can I download Win 10 Pro again?

bimfi said:

. Do I once again download Windows 10 Pro from Microsoft and install on my new build, or do I get the chance to use the Win 10 that I purchased from Best Buy?

It's not entirely clear if you used the key from the Win 10 Pro you purchased to activate the old PC. If you didn't use it then you can use it on the new build.

Yes, download from Microsoft again (you want to install the latest version, not the one you bought two years ago). When you run Setup choose Pro as the edition to install. The key you bought two years ago is valid for Windows 10 Pro, it's not tied to a particular version number - it's as valid for the latest 1709 as it was two years ago for the version 1511 (probably) that you bought.

I guess it's not like the days of Windows XP or 7, where a product key was necessary to prevent pirating?

No, you still need a key, but it seems you have an unused one to hand. Without a key Windows 10 will run, but it will not be activated and will have minor restrictions (can't set desktop wallpaper, for example). It is a violation of the EULA to use an unactivated copy but it won't expire. You'll just keep being nagged until you buy a key.

If you bought a Retail key and did use the key on the old PC, but have now removed Windows 10 and 'factory reset' it to an earlier Windows then it is theoretically possible to transfer the key to a new machine. You may need to phone Microsoft support to get it activated though.

Someone on another forum suggested that I just take my old c:\ drive and put it in my new build. I thought that was a good idea then realized that all of the hardware is different and would confuse the OS.

Windows 10 is surprisingly good at sorting out the drivers when you do something like that. Activation will be a problem though, the new hardware won't match any record on the MS activation servers so Window will not be activated, not without a valid key.

A clean install is probably the easier route.

bimfi said:

Thanks, Bree, for clearing that up. My other concern is for the one receiving my old PC. After I restore it to factory, will they have to put in a product key? I don't remember ever putting one in when I up graded to Windows 10 a long time ago. If they do, I guess they could do like AMDMan2016 and chat with MS support. Again, thanks!

What version of Windows is the factory reset?

An OEM factory reset works differently for 7 or 8.

On an OEM Windows 7 PC supplied with Windows pre-installed there is an SLP code embedded in the bios firmware that activates the restored OEM system image automatically. It's not a key as such, but it is proof of the machine being licensed to run Win7.

For Windows 8/8.1 machines there is a full product key embedded in the firmware. This activates the system after the reset.

In fact, if you clean installed Windows 10 on a PC built for and supplied with Windows 8/8.1 then Setup reads this embedded key to determine which edition to install and treats this as an upgrade of a qualifying version of Windows. This may explain why you didn't need to use your key, the PC qualified for a license in its own right.

bimfi said:

That makes sense. Being so long ago, I think I originally was running win 7 pro, then possibly upgraded to a beta win 8 (?), then upgraded to windows 10, if that makes sense. Like I said it was a while ago. I will play it by ear and see what windows wants to do.

Well, an upgrade of a licensed Windows 7 or 8 to Windows 10 would have given that PC a digital license tied to its unique hardware ID and store on the Microsoft activation servers. It would have its own license for Windows 10. Before you decommission it, check its activation, it would say 'activated with a digital license...'.

If so, you could probably do a clean install of 10 before passing it on. The unused key you bought would be free to use on the new build.