I bought a netbook about 6 months ago. It was my first Windows PC in years. Yes, I ignored the warnings to install a virus scanner, thinking "I'll just surf carefully and I'll be fine". Yes, I got infected within a few weeks.
Let me guess, you were running Windows XP?
Normal use with fully up to date patches (that's how Nimda, amongst others, spread) on Vista (or 7) will net you no malware, unless you stick USB keys into your PC. The problem with USB keys is that you can neatly make your virus executable look like the open folder option at the top (it's social engineering, not a technical flaw). I believe they've modified that in 7.