XP Security Lockdown
While updating a family computer (you know what that's like ... 15 security updates behind), I realized just how important XP SP2 will be (Paul Thurrott's preview is here.) While it can make initial configuration of some net-enabled apps more difficult (you'll have to add the application), in the end there will be less of those annoying popup programs to clean up on peoples' machines.
Update: In the meantime, users may do any number of things to make their pc's more secure (many are outlined here), such as install anti-spyware software (my favorite is Spybot - Search and Destroy), install a hardware firewall (such as a network router), enable ICF (internet connection firewall), and not to open email attachments.