The most open and dangerous part of the majority of computers, is the Internet browser.  Potentially exposed to dangerous code, spyware and phishing attacks on an increasingly regular basis, browser security is of utmost importance.

To ensure the maximum security possible, I would highly recommend using the Mozilla Firefox browser - this provides you with reasonable standard security, regular updates, and a fantastic range of add-ons.  As well as from a security standpoint, I find Mozilla Firefox more productive that the alternatives available.