A good firewall and antivirus program is recommended. There are several good free ones out there. On my XP I use the Filseclab Personal Firewall, a good free firewall from China (the source code for this is for sale for the right price, BTW). A couple of other free ones I would recommend are Privatefirewall, ZoneAlarm, Comodo Firewall, and Online Armor).
I use Avira Antivir Personal Antivirus Free, but some other free ones are Avast, Microsoft Security Essentials, AVG. Some of the better AVs that are not free are Vipre, GData, and Eset 32. I have used all of those AVs and they are all excellent. They are meant to be used one at a time, though, not together, unless they are programmed, like GData, to run two AVs together.
You can also use private browser modes or use TOR, so make you computer location invisible to websites. I use both on my Win 7. You can download the Vidalia package for your particular computer. The Vidalia/TOR package I downloaded did not install on my XP, but you might find a particular one that will.
All of these should keep most of the viruses out of your computer, but also be careful where you surf.
"download malwareybytes, www.malwarebytes.org, run a scan with that its free. it will pick up any Trojans on your computer"
I don't agree with this statement. Malwarebytes is malwarebytes. It will not remove all trojan viruses. It only scan where the antivirus weak point.Just like other tools similar to malwarebytes. But yes, you can scan them using Malwarebytes, they are good. And, you already have a very good antivirus, im using it too. Update your BitDefender and run a full scan. You also can use SuperAntiSpyware as a combo.
I usually don't like Microsoft Product, but Microsoft Security Essentially makes me think difference , they are very light and on top of it , it's free. There is no annoyance message trying to convince you to buy or upgrade.