Paul Boutin has a Good One Over @ Slate in which he has whittled the world of options down to three steps that, on most PCs, can be done in less than 20 minutes.
They're all free, thanks to a mix of promotional offers and hacker idealism. Do all three, and you'll be protected against the most common infections and still be left with time and money for lunch.
1. Set your browser and e-mail security.
2. Get Microsoft's security updates.
3. Check for spyware.
Of course #4 would be buy a Mac or Linux.