This interesting study by a Danish security firm (http://net-security.org/malware_news.php?id=1863) found the main reasons people get viruses is because they don’t update their software.
The main vectors for getting infected are via old versions of Adobe Flash, Adobe PDF Reader, Java and Microsoft Internet Explorer. So if you use these, make sure you keep them up to date.
The conclusion of this study is that as much as 99.8 % of all virus/malware infections caused by commercial exploit kits are a direct result of the lack of updating five specific software packages.
Microsoft also recently published a similar study where they found about 90% of virus infections were through unpatched software.
As they say, prevention is better than a cure. As annoying as it is, it’s safer to keep your software up to date (and less annoying than getting a virus).