T-m & avg
If you dont mind paying for your software I would recommend Trend Micro. Its a great product to protect your computer. Their software offers LOTS of features, and I love it. Also if you ever have a problem their support line is very helpful and knowledgeable.
I also use AVG free, its great too. Though the free version doesnt come with as many features, but its still great for protection.
I have AVG free on my fiancees computer and Trend Micro on mine.
In addition to anti-virus software, I would continue by saying....
Rule of thumb I follow is, dont:
1. do anything,
2. go anywhere,
3. download,
4. open any mail
5. run unknown/untrusted .exe files
...that may be a threat.
Also, stay away from:
* file sharing sites
* illegal sites
* PORN
* "free stuff" sites
* anywhere that seems weird (you know it when you see it)
Staying away from those things will prevent a LOT of problems. Also be careful what emails you open. Even if its from a friend, ask yourself
"how careful is the sender/friend... could they be emailing me something malicious, accidentally?"
Dont use illegal software. If you didnt pay for it, its probably pirated, thus illegal (in many cases, not all). Illegal software is usually full of problems, both legal and virus related.
Also I would highly recommend that you have only ONE anti-virus on your computer. Having more than one will create conflicts between them, causing loads of false positives, among other things.