I use the $30 Zone Alarm Ativirus program and works great all around.. It gives you a pop up window when it see's something that doesn't belong.. Right now it tells me it's defeated 8 external attempts & two of those were malicious attacks. It's also taken down two Trojans. Oh, and I also have it setup to scan my puter when I fire it up. At the end of my puter day I run Crap Cleaner to delete cookies etc... this one is free and works great!
I've had puter probs in the past, no more since installed these two programs!