Updating your PC is often an innocuous process, but those running more recent versions of Windows may find that their latest round of patching included a nice surprise -- a downloader for Windows 10.
The update, KB3035583, weighs in at 493KB and can be found in the "Optional" section of Windows Update (if it's not already installed). Once applied, navigating to
X:\Windows\System32\GWX (where "X" is your OS drive letter) will reveal a number of files and several executables, one of which has the description "Get Windows 10".
The support article for the update makes no mention of its functionality. It's hardly nefarious, so there seems little reason not to supply some details, especially as it's an optional install.
Over at MYCE, Jan Willem Aldershoff has taken a look at the included XML files, if you'd like more analysis. It's sufficient to say it doesn't reveal anything shocking, other than a couple of advertising and telemetry URLs.
We know Windows 10 will be a free update for current OS owners -- even those who acquired it though unofficial channels -- so it makes sense it'd be prepared ahead of time to make the upgrade as painless as possible. This looks like the first step towards that end.
If for some reason the update doesn't sit well with you, MYCE has a guide on how to remove it.