Scan on subst drive does not work

In Windows you can create drives using subst command, for example:

  subst w:  c:\windows

After this command a new drive W: is included in Explorer, and W:\ shows C:\Windows content.

If I scan some file or directory on W: Avast said: “Some files cannot by analized”, but if the same file or directory is selected but usind C:\Windows path it works. For example:

 scan  w:\system   
     said "Some files cannot by analized", procesed files: 1, examined folders: 0

 scan  c:\windows\system
      said "No threat has been detected", procesed files: 31, examined folders: 1

Why ??

Regards.

What version of avast! are we talking about?

Sorry, i forgot this.

avast! Free Antivirus
Program version: 5.0.462
Database: 100331-2

Please post the expanded report in the first case (so that the exact error is visible).