First you need to find any folders called Startup. I find I have 2 of them:

C:\Users\System User\AppData\Roaming\Microsoft\Windows\Start Menu\Startup

[On my computer, the above folder contains 2 shortcuts.]

C:\ProgramData\Microsoft\Windows\Start Menu\Startup

[On my computer, the above folder is empty.]

I think the location of your Startup folders may vary according to what operating system you are using. Also, I think if you have defined any users other than "System User" will have one for each user.

Simply delete any shortcuts you find in your Startup folders. Deleting a shortcut is always safe; it doesn't delete the actual program from your hard drive. You can always recreate any shortcut. Deleting the shortcut will stop the program from being started automatically when you reboot.