Have you looked at this Avast uninstall link
This is from the Avast forum
It is mentioned numerous times
I installed Avast 5 Free Anti-virus on a Windows XP computer with Win7 and did not restart my pc immediately and probably corrupted some files.
Now, the Windows Security Alerts icon constantly appears in the system tray. Windows Security Center under the heading "Virus Protection" indicates that avast! Antivirus reports it might be out of date.
I tried to delete Avast from Add or Remove Programs and only by deleting some keys in the registry associated with it was I able to remove it from the Programs list. However, when I tried to delete Alwil Software and its keys and sub-keys I got this error message: Cannot delete Alwil Software: Error while deleting key. Permission for this key includes Everyone and still it cannot be removed.
When I went attempted to delete Alwil Software in Program Files, I got this message: Cannot delete ashShell.dll. Access is denied. Make sure the disk is not full or write-protected and that the file is not currently in use.
I tried reinstalling Avast 5, but it will not install and shows a bunch of error messages. In the meantime, I installed Bitdefender Free antivirus version and the Window Security Alert keeps showing up in the system tray.