I am toying with the idea of installing BitDefender as a secondary AV, and having it NOT open on Startup.
Free version, no trouble with avast!
But it won't be detected by Security Center either.
It's strange, avast should be recognized immediatly... Are you using the last setup version?
Can you repair your installation?
Go to Control Panel > Add/Remove programs > avast! antivirus > Remove
Then choose Repair function in the popup window (Repair).
You must be connected to the internet while repairing.
If this does not help, can you uninstall / boot / install / boot again?