okay anie !
1- scan your computer, after the scan is over, the error appears. in the window which shows the unscanned files copy the the the path which look like C:\ PROGRAM FILES\ALWIL SOFTWARE\AVAST 4\... \0000: ---> . Open Desktop and paste the path in the address bar.
then
2 - press Ok,
3 - go directly to the file.
4 - right click on it
5 - and scan it, then see if avast is unable to scan it.