Many of the internet attacks these days are DDoS, distributed denial of service attacks, that are carried out by an attacker that has infected hundreds if not thousands of PCs with a trojan designed to flood a target at his command.  This is called a botnet, and the infected computers are called zombies.  These zombie computers can be any computer in the world, the attacker doesn't care who his zombies are, just that they exist.  That means that you could be the unwitting tool of a malicious user.  Trend has released a new tool called RUBotted that runs in the background and looks for signs of zombie infections.  I haven't personally tried this and I know very little about it but it can't hurt to run it once and see if you're infected.  Even if you're not infected, it may not be a bad thing to keep it running in the background to make sure you stay that way.  This could be an option for IT departments that worry about the PCs on their networks from becoming zombied, which is happening more often.

This is by no means a cureall and you should still run your antivirus and antispyware software regularly and use smart downloading practices as a first line of defense.  RUBotted would only be a supplement to catch things your antispyware solution didn't catch.