The main thing is to confirm that avast is actually running.
- What avast processes are running in Task Manager, they begin with ash or asw, see image ?
In previous OSes, the Windows Security Center has been flaky at times (I'm sorry about the technical terms) and either failing to recognise your AV or still recognising your previous AV.
Have (or did) you another Anti-Virus installed in this system, if so what was it and how did you get rid of it ?
Try this, I don't know if this is valid for win7 as I don't use it:
Click Start (or whatever win7 calls it), Run and type CMD.EXE
* Type this command and press Enter:
net stop wscsvc
net stop winmgmt
again you will have to see if this path exists in win7:
* Using Windows Explorer, rename the folder %windir%\System32\Wbem\Repository. (For example, %windir%\System32\Wbem\Repository_bad.). %windir% represents the path to the Windows directory, which is typically C:\Windows.
* Switch to Command Prompt window, and type the following and press ENTER after each line:
net start winmgmt
net start wscsvc
EXIT
Reboot