Whats e best anti virus or a gaming pc which is abt 3yr old..i used it to play multiplayer games?

i tried a lot of freeware n trials but it uses too much memory..u noe hw wen u play online n suddenly ur pc crash or force to shutdown..hate that when it happens im using nod trial version now seems ok but need opinion on these matters..i really2 nub on this..appreciate it guys