One of the things you can't afford to procrastinate about is your antivirus software.
Each of these suites should have its own automatic update functions in place; they are essential for keeping your system armed with the latest malware defenses. Since Microsoft's own Security Essentials updates via Windows Update, you won't need to configure anything else.
As long as you have a decent antivirus suite, you don't need to put much additional effort into staying safe--just don't click dubious links or open questionable attachments. Also, make sure that your e-mail client doesn't automatically display external images (Gmail takes this precaution by default; to re-enable the option temporarily, go to Settings under the General tab), and plug links into Google before clicking them, to confirm that they're legit.
Protect Your PC By Verifying Shortened URLs. To verify shortened URLs (generated by bit.ly, TinyURL, and such) before clicking them, grab the Untiny Greasemonkey script to verify them before clicking. We have plenty of other tips for business users and consumers alike in "Enterprise Security Tips on a Small-Business Budget."