Getting rid of Norton Security Scan

I downloaded this by accident the other day and I have tried to get rid of it through the usual way: control panel, add or remove programs, and delete it. However, every time I turn my computer back on, it’s back! I’ve downloaded one of the norton removal tools (but so many programs were listed none of which were the norton security scan that I don’t know if I got the correct one) but that did nothing. Please help!