This isn't clear cut as far as legalities are concerned. There are arguments to be made for both sides.
You could argue that MS has entered into a legally binding contract. A legally binding contract requires an an offer, acceptance, and consideration amongst other things.
Offer
This wasn't an offer at all. In fact, this was an invitation to treat. MS had no obligation to provide you the subscription for £0.00. When you clicked on the "Place Order" button, it was in MS's discretion whether or not to accept your order. You make the offer to purchase the subscription for £0.00, MS then decides whether it wishes to accept or refuse your offer.
Regarding the survey, you could argue there was no indication on the sign up page or in the T&Cs that a survey was to be filled in, thus it is outside of the contract. MS would obviously argue the opposite.
Acceptance
Because it was an invitation to treat, it is MS who are accepting your order. You could argue that by providing you access to the subscribed service, MS accepted your offer.
Are MS also implicitly accepting that the survey was outside of the contract because they provided you access to the service, even though you didn't fill in the survey? An argument could be made.
Consideration
This is the crux of the matter. Consideration is something that is "valueable" in the eyes of the law. It doesn't have to be monetary but often is. For there to be a legally binding contract, both sides must provide consideration.
MS's consideration = giving you access to their service.
Your consideration = nothing, whether monetary or otherwise.
This is a one-sided bargain. But there are arguments you could make for both sides.
Consumer: Could rely on cases which state that consideration does not have to be adequate. The general rule is freedom of contract. The courts shouldn't interefere with agreements decided by the parties themselves, even if it is a harsh and one-sided agreement (as it is here). This means that even consideration which is nominal (meaning trivial or insignificant) can be valid, thus allowing a one-sided contract to be enforced. Freedom of contract is the reason why courts tend not to look too greatly at whether the consideration given by one side was adequate.
MS: Could rely on cases which state that consideration must be sufficient and of economic value. MS could also argue that consideration was not even nominal (e.g. £0.01), it was actually non-existent (£0.00), therefore no valid contract exists.
Terms and Conditions
Assuming that the court takes the consumer's side and decides that the contract is legally binding, MS may then be able to rely on a clause in the contract.
(Of course, it may be that consideration was not valid and therefore no legally binding contract exists in the first place, which means that you wouldn't even look at the T&Cs)
Remember this page when you signed up? Always worth reading and saving:
That's MS covering their back. You agreed to that term before you placed your order.
However, the double-underlined bit is crucial. A consumer could cite UK legislation which may potentially render that clause unenforceable.
Unfair Contract Terms Act 1977 said:
s 17.
(1)
Any term of a contract which is a consumer contract or a standard form contract
shall have no effect for the purpose of enabling a party to the contract—
(a)
who is in breach of a contractual obligation, to exclude or restrict any liability of his to the consumer or customer in respect of the breach;
(b)
in respect of a contractual obligation, to render no performance, or to render a performance substantially different from that which the consumer or customer reasonably expected from the contract;
if it was not fair and reasonable to incorporate the term in the contract.
Read the bold underlined bits. What that means is, MS can only rely on the highlighted term (which effecitvely allows them to not carry out their side of the bargain) if it is fair an reasonable to include that terms in the contract. This opens up another avenue of arguments and legalities.
Conclusion
The vast majority of those that signed up to this were unaware of any surveys, the linked page did not mention this and nor did the T&Cs. There were no bad intentions for the most part (although a few did abuse the system). MS made a geniune mistake by keeping the link unrestricted and due to this, most people assumed it was a genuine promotion by MS. From a moral point of view, you should not use the keys or downloaded media as it is now clear that it was a genuine mistake. From a legal point of view...not clear. Personally, I will not be using the downloaded media or keys.
And one last point, damages is not the only remedy available. Specific performance is also available (which would require MS to provide the subscription), but this is entirely at the court's discretion.
Edit: The information contained in this post does not constitute legal advice, it is merely my personal opinion. You should seek professional legal advice if you require such advice.