Re: +1 for Linux (And sharing malware in 5 4 3)
I'm pretty sure I remember a torrent-based apt, but it is not the default.
Well there was a thing for Debian called "jigdo" ("Jigsaw Downloader") but it's discontinued and doesn't seem to be a proper peer-to-peer system anyway (more like axel under the hood). IIRC, Warcraft uses an actual p2p system for downloading updates, but I'm not 100% sure on that.
For Debian-based systems (with .deb files and all the server architecture that goes with it) it should actually be very easy to employ a p2p system (or plain local caching) within a LAN. Provided your DNS isn't being tampered with and you can download the list of packages and .deb file checksums (of which there are two per file) then you should be able to trust any .deb file you download provided it matches the checksums. I'm not sure if Windows has ever supported the idea of downloading an update once and serving it locally (unless you do it yourself using a transparent proxy, but not sure if that works) but it's always struck me as something that would be nearly essential in any decent-sized organisation.
(anon due to this mostly being ignorant speculation)