At work, I have been using my laptop for ~3.5 years now, running Windows 7, without reinstalling once. If you take some care, don't just install any random piece of software you run across, only download software from trustworthy sources, etc., it is possible to keep a system free of malware.

Just to be clear, at home, I run GNU/Linux, too. But it's not like catching a virus is inevitable fate on Windows.