+1 for Kaspersky
I love it because its "light touch"... but very effective. Some of the other ones recommended above I have found to be very "bloaty"... and just wont shut up... They seem to feel like they have to tell you they started up... that they updated definitions...that they identified that your internet browser history is not clear!!!
Kaspersky just seems to queitly get on with it. Its ad blocker is great... I dont realise just how much crap comes at you on some web sites until I visit on another comp... I really appreciate that
And also... I NEVER notice it doing a scan....may be beacuse I leave my comp on all day - prob plenty of idle time for it to do it... but I never notcie it impacting through scans.
I am careful - so dont expect to get warnings... but the very few I have had gives me confidence it doing a good job.
And it never gets in the way of any games.