Enabling Steam at startup is useful because any updates are loaded. On the other hand, Steam auto startup slows down the computer's startup and we find ourselves waiting for Steam to finish its work while we have something more urgent to do.

Here's how to disable Steam Autostart on Windows.

Click on the Steam icon found on the desktop below

Click on Steam in the upper left corner and a popup will open

Click on Settings

Click on Interface

Uncheck "StartSteam when my computer starts"

Confirm by clicking OK

Done!