BEFORE you load a new anti-virus software DELETE the old one. Otherwise there will be conflict and you could get all manner of odd things happening.
For something free, use AVG, as recommended elsewhere. Don't use Norton, it's clunky and slow. After many years I find PCtools the best. There is a free version of that but the professional version isn't too expensive, It's much faster than Norton when doing the daily scan.
What do you mean you don't scan every day!
