I have this problem too - when Avast! 5 was first installed (Windows 7 64-bit), it was recognised by security center; however (7 weeks later) the system suddenly announced that there was no anti-virus software detected. I have tried the repository files "fix" as suggested but this had no effect (the repository was declared to be "consistent"). This was on a new laptop which had a trial version of McAfee pre-installed, which was removed using the proper removal tool prior to installing Avast! 5. As I said above, though, Avast! was recognised ok for the first 7 weeks. Automatic updates to Windows and Avast! are working correctly.
.NET framework 4.0 was the only new software installed anywhere near the time of the problem.
Avast! 5 itself appears to be working fine (as far as I can tell) - this seems to be a false report for some reason. Could a recent windows update or Avast! update be responsible? I see that Windows Update is now showing Microsoft's own "Security Essentials" package as a recommended update, is this coincidence?