How to prevent being infected
It all boils down to being careful. Do not open an email attachment you are not 100% sure of! If someone sends you a file and you are not sure what kind of file it is, email back and ask them BEFORE you open it. If you are the person that likes to send funny programs via email, well STOP IT. That harmless cartoon.exe file could be a Trojan or Virus in disguise. While you are looking at a funny screen, it could be trashing your system. If you must send it, find the original web page and send the link and not the program. If you cannot find a web page for it... well you have your answer.
If you need to send other files, try zipping them into an compressed archive before sending. This makes it easier to scan for a virus at the other end. Picture files like jpg and gif files are safe to send as are mpg, mp3 and avi files. They are already compressed and to date, no one has been able to infect them with a virus.
Keeping your Windows Operating System up to date is also a big help. This costs you nothing but download time from Microsoft. Go to the Windows Update page and follow the on screen instructions. Internet Explorer and Outlook Express have some very dangerous vulnerabilities unless you update your software. Running older version of software can in itself open you up to vulnerabilities.
So please practice safe computing and install some software to protect yourself and others. As for advice on what to install, well in the authors opinion, ask your friends or your computer mentor. I have always found it best to use what your friends are using so you can get help that much easier (and this advice can be applied to almost anything computer related!).
And check for updates at least once a week!