A security monitor will tell you if your computer's protection (such as antivirus or antispyware) is out-of-date or not working.
Enable a Security Monitor
There are several security monitors available. Windows XP service pack 2 has a security center (accessible through the control panel). When enabled, it will alert you if your firewall or antivirus protection are disabled or out-of-date. It can monitor several, but not all, 3rd party antivirus scanners and firewalls.
ZoneAlarm's firewall has antivirus monitoring which supports most 3rd party scanners.