alt Hacker NewsThose aspects of contract law mean that if MS included "you owe us your first born child" or "if you have not uninstalled this operating system within 2 weeks of installation, you owe Microsoft an additional one million dollars" then that clause wouldn't be valid.
They don't however mean that MS choosing to put adverts all over Windows is illegal, or a breach of the contract, just because users would prefer the OS be ad-free. The EU could legislate in various ways that would mean MS had to stop doing so, but they haven't yet and there's no aspect of general contracts law currently that prevents it.
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If you bought and paid something (not a subscription) that was ad-free and then all of a sudden in a mandatory update you start to get ads, well, maybe someone already tried and failed to sue MS but personally seems pretty predatory.
Many countries have laws against "hidden defects".
One could argue that adding ads after some time from a system putchased without ads throuh updates is a defect that has been hidden at purchase time.