The apple update process checks for Quicktime and iTunes updates from time to time. Apple, in its wisdom, has now decided to foist Safari on PC users, in what could be considered an underhand manner. Next time there's an iTunes update, unless you look carefully it will install Safari, too: there's a pre-ticked box on the download page for Safari.
Some people are getting quite upset by it: people accept frequent online checks for software bugfixes, security fixes and updates, on the understanding that there won't be an unexpected download of unwanted software. If the latter method becomes common, people will turn off automatic checks for updates. Apple has really dropped the ball on this one.
FYI, Safari's adequate on a Mac but it's a bit of an also-ran on the PC.