Back in the old days, there were a lot of places an application could hook itself to run at startup. You had to check the registry in more than one place, as well as your start menu. With Windows Vista, there’s a built-in panel that handles all that for you.
To quickly get to this panel, you can open Control Panel, and then type “startup” into the search box. The link for “Stop a program from running at startup” shows up immediately. You could have also opened Windows Defender and fooled around with the menus until you find the right spot, but this seems quicker to me.