Same problem here. I had had OneCare on Vista Business, and suddently the Security Center started reporting OneCare didn't report to it in correct format. I removed OneCare and installed Avast. It didn't help though. Instead of the incorrect format message I began receiving Avast antivirus is turned off. Clicking the Turn On button forced Avas to update itself but didn't change the message.
Amazingly, I installed Windows 7 on the same computer as a double boot soon after the problem struck my Vista, and the same problem showed on the Windows 7 after a couple of hours of using it. But antivirus was Panda Beta this time.
I have too more Vista Business computes on my home network, and they showed no sign of such problem.