First of all you must tell us which OS are you using it cos otherwise this will lead us to no end.As for window,I recommend you to grap Kaspersky AV which is really the best among so many AV i have been testing it reliablility.One thing bad a Kaspersky is that once it start scanning,it eat up lot of your system resources.Which mean that you can only do other things like web browsing after your AV scan.