its absolutely legal if you have a valid product key
Torrenting Windows? I don't think so in the US. Uploading/seeding it to a swarm is distribution of copyrighted material.
I haven't seen anyone get sued over it for Windows, but I'm out $1500 (attorney's fees and settlement, because it was cheaper than continuing to pay the attorney) after (I'm guessing) one of my sister's kids downloaded some Adam Sandler movie on my connection. I hadn't heard of it until Comcast sent the subpoena notice, and it apparently did very poorly in theaters. Maybe they could have had the case continued, but they never provided anything to support that the IP address had been used to share copyrighted material, only that some device at the address was a member of the swarm. (TL;DR: copyright trolls suck)
DigitalRiver used to host Windows 7 installer images via http, which wouldn't involve redistribution on the downloader's part.
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