If an online intruder has infiltrated your Windows PC, you may notice recurring slowdowns of e-mail and Web browsing, or you may notice nothing at all. PC users must shore up defenses on three fronts:

Operating system vulnerabilities. Always use a personal firewall and keep security patches up to date.

E-mail viruses. Distrust all attachments. If you doubt it, delete it. Subscribe to anti-virus software, such as Norton AntiVirus, McAfee VirusScan or ZoneAlarm Security Suite. Keep the subscription current and set it to automatically check for updates.

Spyware. Consider switching from Internet Explorer, a sieve for spyware, to the Mozilla Firefox browser or the Opera browser. Both are free and can be downloaded, respectively, from mozilla.org or opera.com.
If you continue using Explorer, set security settings to high and use anti-spyware software.