I replaced the hard drive on my HP Pavillion DM4 and dont have the product key for windows.

[Poboboy]

I need the original product key in order to upgrade to Windows 10

[CherylG]

@Poboboy 

You should have a COA attached to bottom of laptop with Windows 7 license key. If you had upgraded to Windows 10 previously you won't need the license key- you were assigned a digital license key stored on Microsoft servers along with hardware id of your laptop. Simply install Windows 10 and when asked for key select "I don't have one". It will install and should activate once you go online.

[Poboboy]

Yes I have the original sticker with the code on the back of the laptop but windows does not recognize it and I don’t have windows 10 as I replaced the hard drive after the free offer. I also did not have an windows 7 disk as it came pre installed and the copy I bought is not genuine. Now what?

[WAWood]

@Poboboy 

Just so you know, the Win10 Upgrade expects to find an activated copy of Windows already present on the PC -- as it then "harvests" the product key of that Windows copy to reuse for Win10.

As long as you're doing an in-place upgrade of an existing copy of Windows, as @CherylG indicated, once you are done with the Upgrade and go online, your PC reactivate automatically.

[CherylG]

@Poboboy 

I am 99.9% sure that you will have no problem doing a clean install of Windows 10 with no OS activated if you previously upgraded and activated Windows 10. Your previous Windows 7 license key was converted to a digital key for Windows 10 and is stored on Microsoft servers along with the hardware id of your laptop. Make the usb or dvd using Media Creation Tool and boot with it. People do this almost daily.

https://www.microsoft.com/en-us/software-download/windows10

Select Download tool now,save it,then Run.
If you agree to the license terms, select Accept.
On the What do you want to do? page, select Create installation media for another PC, and then select Next.
Select the language, edition, and architecture (64-bit or 32-bit) for Windows 10.
The Guide here shows the steps:
https://www.tenforums.com/tutorials/2376-create-bootable-usb-flash-drive-install-windows-10-a.html
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When the flash drive is prepared insert it in a usb port. Power on the laptop and immediately start tapping Esc key. From the menu tap F9 key and use the arrow keys to move to and select usb. Tap Enter key.
When installing and you go through first few screens- choose Custom and select each partition and delete each (if you have more than one).Install on the unallocated space left from that process.Continue with installation. When it asks  just choose "I don't have a key". It will install and should activate once you are online and Windows checks the server at Microsoft. Usually within 3 days you are activated.