Windows 7 OEM to Windows 10 activation

Was the new replacement motherboard a true Dell-Alienware x51-R2 motherboard?

Does the PC still have Dell or AW logo when it boots up?

No, the motherboard brand is not affiliated to Dell.

You cannot pass activation from one machine to another.

No, the motherboard brand is not affiliated to Dell.

That copy of OEM Windows (and activation key) will only run legit on a true Alienware X51-R2 (motherboard).

You cannot pass activation from one machine to another.

• The install media on the other hand, if it's branded by Alienware, then it, and only it, would be bound and only able to install Windows on another Alienware.  You can't, for instance, utilize an Alienware branded Windows 8 install disk to install Windows 8 on an XPS, and vice versa.  This is due to the OEM key, and a few other related options, that's been integrated into every branded install disk

2 years ago I bought an Alienware X51 R2 with the OEM Windows 7, 6 months ago I upgraded my pc to windows 10. A few days ago I upgraded my hardware and got a new motherboard, case,  PSU and Networkcard and now windows is asking me to activate windows with a product key. I know theres a product key on the back of my pc but its not valid, it only works for windows 7.  Where do I get my valid activation key?   I also posted this on the microsoft forum.

I'm assuming you took the free upgrade to Windows 10?  Grab a spare HDD, install it, and install Windows 7 onto it.  Activate Windows 7 with the COA product key, and then take the Windows 10 upgrade.  Once you've verified your hardware ID shows activated (i.e. Windows is Activated) in Windows 10, you can pull the spare hdd and boot from your old hdd.

• If the only install media you have for Windows 7 is the Alienware branded install media, you'll need to download an install media iso from microsoft.com
• Once you're activated again, you'll need to remove your old PC from your activated devices on account.live.com

Microsoft changed their licensing structure to allow for Windows 10 activation via Windows 7/8 product keys (however I'm not sure if they implemented this now, or if they were going to implement it in the near future).  You should be able to find an article about this from Google, as a few pieces were written about this change several weeks back.  

• Provided your Windows COA (Certificate of Authenticity) is still legible, you can use that product key.  If it's not, there's a hyphenated hexadecimal group in smaller print somewhere on the COA and this serial can be given to Microsoft Activation Support to have them issue you a new product activation key.

I always thought the COA was 'locked' to the machine legally?  Of course i've "transferred' a COA from one machine to another if the first one died or whatever, but I always thought it was a grey area *shrug*.

I'm not sure where you're pulling that information from... nor how someone could come to such a conclusion, or believe a legally binding EULA is a "grey area"

• The most obvious being, Microsoft and Dell are two completely different companies.
• A consumer purchases hardware from computer manufacturers, such as Dell, who in turn sell the consumer a consumer license Microsoft has authorized them to sell.  It's no different than purchasing an Adobe product with your computer purchase, an antivirus, or any other piece of software.
  • Microsoft strictly regulates the terms under which an OEM can sell licenses for a specific version of Windows 
  • U.S. Federal IP laws state [paraphrasing] any device sold with a paid OS must include a COA (this serves a multitude of purposes, two of which are protecting consumers from trade abuses by a hardware manufacturer and protecting the consumer against being sold a fraudulent license)
    • When you buy a new device with Windows pre-installed, the activation hasn't taken place via the product key on the COA, but via the OEM's volume activation keys they're granted by their contract with Microsoft.

• Most importantly, what governs what an end user can and cannot do with a license is spelled out extremely clearly in the EULA.  When has someone ever seen Dell's name, let alone any hardware manufacturer's, anywhere inside of the Windows EULA?

I appreciate the fact no two people hear, see, or read something the same way... however, it does a disservice to all when individuals, any individual, attempts to explain a legal concept without ever bothering to read the concept they're attempting to speak to.  Whether it's smartphone warranties and users arguing to high heaven it voids the warranty if you root your device, when the warranty doesn't say so, or users stating a Windows license is tied to a motherboard... Users specifying this specific type of legal information (as warranties and EULAs are legally binding contracts between the manufacturer/developer and the end user) need to be 100% certain they're giving the correct, factual information before stating it.  

• I don't say this out of hubris or condemnation, but out of the simple fact once you say it online, it stays online and it's near impossible to get rid of.  It's easiest to show this via the smartphone warranty example above:
  • Users who've never read a smartphone hardware warranty have stated millions of times to other users "rooting a device voids your warranty" because someone said it to them or some store associate said so, and even though it's not true for most smartphones, a substantial amount of end users now believes information that not only isn't factually accurate, it's flat out wrong.  Even worse, many users shown in black and white, verbatim what the warranty actually says will still argue to high heaven it's wrong and rooting voids the warranty.
• The point is, facts matter... even more so when it comes to the legalities of what an end user can or cannot due with the product(s) they own.