I personally use BitDefender and prefer that one. It can be a bit of a resource hog when running an antivirus scan, but I've just set it to run them manually and then I run them overnight while I'm sleeping (I'd set it up to do it automatically, but I don't always go to bed at the same time and I don't like to leave my laptop running all the time). The rest of the time, BitDefender has been great and just to attest to it, it doesn't even bog down my netbook.
I also tried Kaspersky and wasn't impressed. It's one of the top rated Antivirus programs along with BitDefender, but I found it did have impacts on performance, and I wasn't too impressed with how deeply it sets itself in the computer filesystems. Makes it hard to remove it. I also wasn't a fan as I typically prefer systems that give me more control over what's going on.
While Norton has really improved, they've got a long way to come again before I'd ever use them again. Their reputation was hit pretty bad due to mediocre scanning abilities and serious system resource issues. Supposedly the new version is much better, but the damage has been done.
