I agree with just about all the replies here.
My suite of programs to install on customers' computers (as well as my own) is:
Zone Alarm firewall
Spybot Search and Destroy (up to 1.5 now)
For my own email, I use Thunderbird (free, open source) and it works on both the Linux box and the Windows box. It will not allow an html bomb to take over your machine like Outlook Express.
I don't use Zone Alarm on Vista because the Windows firewall is better than XP's. A lot of my customers have a hard time with Zone Alarm because it can be confusing.
I'm waiting for an OS to come out that doesn't require constant updating of every single program. It's really frustrating trying to fix one, especially one that's used with dial-up, while the computer is trying to apply 8 Windows Updates, java update, Firefox update, Windows Installer update and seventy-six other updates simultaneously.
My, but I do love Linux.
I dropped Ad-Aware after the recent release .... seemed to be a resource-hog. I also use AVG Spyware, which is free for personal use.