Something you need to understand, OP, there's a difference between a preventative anti-virus program and a malware removal tool for use after you're already infected. Norton, AVG, Avast, Avira, etc. are primarily examples of the former (although they do have limited removal tools), while Malwarebytes, Windows Defender, and SuperAntiSpyware are examples of the latter. You normally want one of each, because the active anti-virus is a good first line of defense, but they're rarely very good at removing things once you are infected, while the removal tools are great at that. They also only run when you tell them to, and in the free versions usually don't have a way of scheduling scans, so they don't eat up memory unless you're actively using them, and they won't conflict with your antivirus. Or each other, for that matter.
DoPo said:
I'd go with Microsoft Security Essentials because it doesn't cost anything and it's decent. Other decent no cost alternatives are Avast!, Avira Antivir, and AVG and I'd probably recommend them in that order but they should all be equally good.
If you are looking for something paid, though, I'd second ESET NOD 32 - it's lightweight and quite good at what it does, but it's...well, has a cost in money.
Good list here, except that Avast! is the most memory heavy of the three, from what I remember, so if that's an important consideration for you, I'd go with Avira instead. It's what I use on my crappy old laptop, specifically because I don't want it eating up resources. Also you'll want a good removal tool. You can go with MSE, but Malwarebytes is kind of the standard. I haven't found a virus yet that I can't clear out with Malwarebytes and, if necessary a combination of safemode, Rkill (which kills virus processes that may block Malwarebytes from starting) and the Kaspersky rootkit removal tool.
Also I'm almost certain Microsoft Security Essentials is just a removal tool that you can set to scan on a schedule, so if you want active protection you'll need something else.
