What happened? The system time and date was incorrect or any other thing happened?
I’m assuming that it was in fact the deletion of the Repository in the above link that restored the security center back to health.
Unfortunately we (I) haven’t got a clue why it happens just clearing the repository forces the security center to re build it, which seems to work in detecting AVs correctly.
I would however, say you need to look at a third party firewall to protect against unauthorised outbound connections.
Whilst the windows XP firewall is usually good at keeping your ports stealthed (hidden) it provides no outbound protection and you should consider a third party firewall.
Any malware that manages to get past your defences will have free reign to connect to the internet to either download more of the same, pass your personal data (sensitive or otherwise, user names, passwords, keylogger retrieved data, etc.) or open a backdoor to your computer, so outbound protection is essential.
- Zone Alarm free http://www.zonelabs.com works fine with avast and has a reasonably friendly user interface, however, the free version is becoming bloated with trial ware. There are others, Comodo, PCTools Firewall Plus, Sunbelt Kerio, Jetico, etc.