Hello.
I updated avast to 4.8.1195 and now avast gives this kind of information:
13.5.2008 20:40:28 SYSTEM 1508 Automatic rootkit scan was not started as it didn’t complete successfully during the last run.
This doesn’t appear after every boot and it isn’t marked as a warning or error.
And I’m using Windows Vista Home Premium SP1.
EDIT: Now I know why this happens, it takes about 15 minutes to complete “aswar” scan.