The F-Prot Antivirus tips list
Make sure that all your security software is up to date. Turn on all automatic updaters and regularly check to make sure.
Keep your operating system updated with the latest patches. This applies equally to all platforms, Microsoft Windows, GNU/Linux and the BSDs.
Only run applications downloaded over the internet if you're absolutely sure of it's origin. Even otherwise legitimate applications can be tampered with and redistributed by criminals.
Don't trust anything that comes over e-mail, even if you know the sender. If you get an e-mail with an attachment that you weren't expecting, get in touch with the sender and make sure it's authentic before you open the file.
Run your internet banking and other sensitive web work in a separate browser from your day-to-day browser. Use something like Mozilla's WebRunner or simply do your banking in IE if you browse in Firefox or vice versa.
Keep all your browser plugins updated. Apple Quicktime, Adobe Flash and the Adobe Acrobat PDF Reader have all had serious security flaws over the last few months, many of which could lead to a total compromise of your computer . Always keep them updated or uninstall them completely.
Be up to date and be safe. Don't trust files of the internet unless they are from reputable sites and sources.
Keep all that in mind and it's simple to be secure.