I have seen people who daily scan their computer for viruses. In my opinion there is no use of scanning the PC every time you start it. It is just a wastage of time and processing power.
Okay, let us put it this way,
When do you go to a doctor for a checkup?
Only when you feel ill.
Similarly, scan your computer only when you feel that something is wrong with it. If you scan your PC daily, it is like going to a doctor for a checkup everyday even when you are healthy. Its common sense, isn't it?