Thanks a lot for the info. I'm afraid it shows that your Avast is really using virus definitions from 6th February - any later are incompletely extracted on disk. That's also why they are not removed - Avast always removes the definitions older than the ones it's currently using...
Could I please ask you for one more file? Normally this does the support tool, but that's not working on your machine. Problem is, I honestly don't know where to find it on your machine... you having Italian Windows, plus the installation is updated from an even older version of Avast (so it's "Alwil Software", not "Avast Software" that the new versions got).
The file is called
Update.log and it's in a folder which, on English Windows XP, is called C:\Documents and Settings\All Users\Application Data
In that folder, it may be in AVAST Software\Persistent Data\Avast\Logs... but for you, it may be Alwil Software instead, and I'm not sure if Persistent Data will be there.
Maybe you could use Windows Search (Win+F) to find the location?
There's one thing I would try to fix the problem - download the full virus definition update from
https://www.avast.com/download-update and run it... chances are it may help.
Sorry for the troubles.