Those 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.