I’ve been using my current laptop for about three years now. I gave my old laptop to my wife when I got my current one. She doesn’t do much except for shopping and writing emails to a few friends.
I had installed some applications on that laptop before I passed it off to her and she recently asked me to remove them so that it would start up faster and free up some disk space. I did that, and then I defragmented the hard drive. It was working much better.
Just for the fun of it, I then decided to run Trend Micro’s Hijack This on that computer, along with Sunbelt Software’s VIPRE Anti-virus / Anti-Spyware. It didn’t find a single thing on it that was bad (outside of a few cookies, which aren’t really that big a deal from my perspective. I just delete them and move on).
The key here, I think, is that my wife doesn’t go off surfing to the four corners of the Internet. She goes to sites that she knows and trusts. She only opens emails from people she knows and trusts. She also trusts her “sixth sense”, and when something seems to be a bit questionable, she either ignores it or asks me. And that’s why I think her computer was so clean, even though I hadn’t checked it for quite some time.
Oh, and I don’t run anti-virus software on her computer, either — and you don’t have to, either, presuming you follow her best practices and have a certified computer security pro in the house to turn to when you have problems…
So stay away from those sites where you don’t really need to go. It will help ensure that your computer stays malware free.
Oh, and this is one of those sites you should visit… 
