One of the simplest but most important security precautions you should take is to keep your PC's software up-to-date. I'm not talking exclusively about Windows here: Adobe, Apple, Mozilla, and other software makers periodically release fixes for various bugs and security flaws. Cybercriminals commonly exploit known vulnerabilities, and Adobe Reader is a constant target of such assaults.
Not infrequently, the latest version of a popular program introduces entirely new security features. For example, Adobe Reader X, the newest version of the company's PDF reader, uses something called Protected Mode to shut down malware attacks. If you still use an earlier version of Adobe Reader, you aren't benefiting from Reader X's security enhancements.
Most major commercial software packages come with some sort of automatic updating feature that will inform you when a new update is available. Don't ignore these messages; install updates as soon as you can when you're prompted to do so. It's a little bit of a hassle, but it can prevent major headaches later on.
For laptops, file-syncing services offer a convenient way to keep your system up-to-date.