Personally, and I presume we're talking about a PC, I've always used
Norton Internet Security (
but not Norton 360 - you don't need the extra bloat!).
I know it's not everyone's cup of tea though so you could also look at AVG, McAfee, and ESET (which I understand comes highly rated).
I'm sure a few other people can chip-in with recommendations too.
Cheers,
Shaun
