Step 4: On the popup window, choose the General tab and uncheck the option that Start Dropbox on system startup and click OK to save the change. Step 3: Click Preferences in the context menu to open a Dropbox Preference window. Step 2: Click the icon of setting on the top right corner. Step 1: Click the icon of Dropbox on the bottom right corner of taskbar. Method 3: Stop by System Startup Settings.Method 1: Stop by Preferences of Dropbox.To fix it, here we will give you three methods to stop Dropbox from opening on system startup. However, sometimes there is a problem that this application starts automatically on system startup. As a cloud storage service, Dropbox offers computer applications for us to use.