you can not scan websites. This is also not needed, as it would make using the Internet frustrating.
you can check your PC regularly and let it alarm you when it thinks something is wrong.
You can also read what your PC tells you. Yes - those little fast clicked popup's do have a certain function. And those dialogs while installing are not just there to fill some space and to keep you busy.
A computer is a small, dumb machine. It actual needs people to tell him what to do and how to do it. And if it does not how to do it, it just stops.
Even our cat is far more intelligent.
regards
Torsten