1. Think before you click on email links or attachments.
Don't click on links in emails from people you don't know, and Don't click on links from people you do know that you aren't expecting or have an odd sounding text in the message. Read that again, it's very important to follow.
Why? Because this is the #1 way for bad guys to own your computer.
2. Keep Adobe PDF reader up to date. If it's been a week since you've updated, check for updates right now.
Go here for Adobe Reader updates: http://get.adobe.com/reader/
Why? The PDF document format includes active content and that means bad guys can hide their programs in pdf documents.
3. Don't use Internet Explorer (IE). Use a widely used web browser alternative like Firefox.
Find it here: https://www.mozilla.com/en-US/firefox/new/
Why? IE is highly targeted by the bad guys and it has a long and continuing history of serious software vulnerablities that allow the bad guys to use your computer in any way they want, that's a bad thing.