Windows 7 product key does not work on Windows 10

I just did a fresh Window 10 installation from .iso files on the computer I was running Windows 7 on before. I have the original Windows 7 DVD's, with a valid key that I was using before.

Now I tried to enter the key of my Windows 7 DVD, but Windows 10 says it's not valid.

As far as I knew we were able to use those Win 7 keys on Win 10. What is wrong?

Unfortunately, you must upgrade an existing installation before you are able to do a clean install of Windows 10. It is only once an upgrade has been done that you have a Windows 10 license at all.

After an upgrade has been performed that PC is licensed to use Windows 10 and a clean install can be done. During the clean install the product key entry should be skipped (multiple times if required) - the PC will still automatically activate when installation is complete.

From what I understand Windows considers the motherboard to be the PC for the purposes of licensing, as long as the motherboard is the same the PC is licensed for Windows 10

For more detailed information on performing a clean install, see this article.

In light of some of the confusion around this it's worth pointing out that although a lot of the Windows 10 licensing messaging has been unclear, the upgrade first requirement for clean installs has been something Microsoft have been talking about for as long as they have been talking of Windows 10 clean installs - so the system is "working as intended". It is, of course, entirely possible that Microsoft will create a new system for this soon, but for now this appears to be the only option.

Note that currently some people are having trouble activating Windows 10 despite having legitimate versions - these problems seem to resolve themselves in time

Edit: Removed unnecessary information about retrieving product keys

The rules are changing...

Up til now, to qualify for your free upgrade to Windows 10, you were required to successfully complete an upgrade on a qualifying machine before you could then perform a clean install of Windows 10.

As from Insider Preview Build 10565 you can now activate using an existing Windows 7, 8 or 8.1 key which was previously used on that machine.

From the Windows blog by Gabe Aul - Announcing Windows 10 Insider Preview Build 10565

Device activation improvements: We have received a lot of feedback from Insiders on making it easier to activate Windows 10 on devices that take advantage of the free upgrade offer to genuine Windows by using existing Windows 7, Windows 8 or Windows 8.1 product keys. If you install this build of the Windows 10 Insider Preview on a PC and it doesn’t automatically activate, you can enter the product key from Windows 7, Windows 8 or Windows 8.1 used to activate the prior Windows version on the same device to activate Windows 10 by going to Settings > Update & security > Activation and selecting Change Product Key.

If you do a clean install of Windows 10 by booting off the media, you can also enter the product key from prior Windows versions on qualifying devices during setup. Refer to the Insider Hub for more information on these activation improvements including requirements.

EDIT -
..and breaking news [which I hope to keep updated with more official sources, once known] which includes the same activation information
Windows 10 Fall Update is Set for November Release

Earlier this month I did a clean install of Windows 10 Pro build 1067 on a Hewlett Packard G6 laptop, using the Windows 7 key off the COA when prompted during install.

I also have a Lenovo laptop that it looks like I'm going to have to start over on because it's rejecting the COA Win 7 key after Windows 10 is installed.

Microsoft is still accepting Windows 7 and Windows 8 keys for 10, but for some reason only during install - though while searching for if it's possible to make it take the key post-install - it looks as though MS is not accepting ALL Win 7 COA keys.

So if you have a computer that came with an OEM install of 7, 8 or 8.1, try the COA key while installing 10. It'll either work, or not. If it works you save the bother of installing Windows twice then cleaning up after the upgrade, or wiping it and installing Windows a 3rd time.