Yes, it is a part of avast!.
If Vista is reporting it as "unknown publisher", you are missing some certificate updates (in Windows).
I suggest the following: browse to avast! folder in Windows Explorer, rightclick the VisthAux.exe file, switch to "Digital Signatures" page, select the signature in the list and click on "Details".
That should validate the signature (does it?) - and possibly also download the certificate update.