Tomorrow, millions of Windows 7 and 8 users will be availing their free upgrade to Windows 10 and it seems that in order to mitigate server load and allow for a smooth and fast upgrade experience, Microsoft has already started pushing the necessary files to users' computers through Windows Update.
According to reports by some users, a $windows.~BT folder can be found in the operating system partition. It amounts to around 3GB and most probably contains the installation files for the upgrade. The upgrade will still take place tomorrow, when the operating system officially launches; only the files have been downloaded right now.
Windows 10 is Microsoft's upcoming - and highly favored - successor to Windows 8, featuring a more familiar experience to Windows 7 users who found Windows 8's UI to be too much of a departure. It also brings with it Cortana - your new personal assistant - a new browser, redesigned and improved applications and much, much more.