If you are running a Microsoft Windows operating system, and since MS owns 90%+ of the market share you probably are, then you have to worry about viruses. It is sad that there are bad people out there who waste their time and talent developing programs that will hurt the computers of other people, but that is the world in which we live.
When it comes to viruses the scariest position to be in is one where you are directly connected to the Internet. Most of the time this means you have dial-up Internet, but in some cases broadband service also puts your computer in direct contact with the Internet. If that is the case, then you should probably go with full strength anti-virus software from the likes of Norton or McAfee.
However, if you have a router between you and your computer then I recommend you save the $50 you might have to plunk down to get anti-virus software and go with the free version from Grisoft called AVG. Here is a link to download the software through CNet's download.com.
There are two reasons I recommend you use this software if you are behind a router. The first is that the router is actually providing you a lot of protection from the bad guys out there. A significant portion of viruses out there utilize known vulnerabilities in the operating system, but in order to exploit those vulnerabilities they have to be able to get to your computer and with that router sitting between your computer and the Internet those options are greatly reduced.
The second reason I recommend this software is because it is really light weight. Not only does it drive me crazy that you have to upgrade the paid for anti-virus packages every year, they have been getting bigger and more intrusive every year. I have used both Norton and McAfee over the years, neither really seeming to have a huge advantage over the other, but last year I gave up on both of them. AVG offers a free version of their software that is limited only to anti-virus protection, which is all I want. I don't need a lot of other fancy things that both Norton and McAfee seem to think I need, wasting a lot of valuable CPU cycles in the process.
In the year I have used AVG, none of my 5 computers running Windows have had any virus issues. In fact, since they are all behind a router, there hasn't even been a virus detected or removed. Think about it, when is the last time your anti-virus software warned you that it detected a virus? AVG may not catch everything, it is true. You may be a bit more exposed using AVG than you would be running Norton or McAfee, but so long as you use some good sense with email (don't open those attachments from people you don't know) and browsing (stick to the sites you know) then it seems like you can save some cash and have your computer run better.