How you approach Virus Protection for your computer is best summed up as a matter of Risk Management.
Every time you interact with the internet, whether browsing websites or downloading emails, your computer is at risk. How high that risk is depends on where you go and what you do while on the net. Downloading music can be high risk, depending on where the download is coming from. Even Facebook etc is being targetted by Virus writers.
You should consider such activities as Internet Banking as being High Rick, not because you might get a virus, but because of the implications if you did get a virus that hacks your passwords etc
The Rule of Thumb is, the higher the risk the better your Anti-virus measures should be. Free Antivirus programs we consider to be appropriate for Lower Risk net use. For Higher Risk net use, you should consider use a "bought" antivirus program such as ESET (PCPatch's recommended choice)
Here are links to some software we find useful in dealing with virus infections. The first thing you should ensure is your windows is completely up to date with the latest windows updates as some of these updates are almost certainly needed for antivirus programs to work effectively.