Steam is still twitchy after all these years? Damn, Steam used to piss me off back in the day when I played CS. I am baffled they haven't made it more stable.
I'm still a Kaspersky fan myself. But Norton 360 has come a long way and runs pretty decent. If you strive for free software... Avast isn't too bad (like Craaig suggested) I had it on one of my computers for about 2 years, but then a virus squeaked by and I wasn't too impressed. But, can't really beat it when it is free.