I was wondering what files Steam (the digital distribution platform) installs and where on a Windows (7) PC. Does it add anything that runs at start-up or add any files to important system folders?
The default Steam installation directory is
\Program Files\Steam. (Steam is not 64-bit, so it will install in
Program Files (x86) if you have a 64-bit OS).
It also registers a Windows Service with the executable
\Program Files\Common Files\Steam\SteamService.exe
It doesn't add anything else to other directories. All the content (e.g. the games) you install through it will be placed in
\Program Files\Steam\SteamApps, but might itself add additional files in other places.
The default is to run Steam at start-up, but this is easily changed in the options, or at install time. You may want to check the Services Control Panel (Win + R + "services.msc" + Enter) afterwards to verify that Steam Client Service is also disabled.
Note: Steam itself exposes a critical vulnerability in changing folder permissions of its
\Program Files\Steam directory, allowing all unprivileged users to replace, modify and/or remove Steams core binaries.
It does install a service and runs on startup by default, though it is easily changed in the options (or you can do it manually, it doesn't restore the autostart if you remove it). The default install location is "Steam" under your default 32-bit Program Files folder, e.g.
C:\Program Files (x86)\Steam\ on 64-bit Windows.
I don't think it puts anything in system folders, but I don't know. It surely doesn't modify anything important.