The default location is C:\Program Files\Alwil Software\Avast4 I don’t know if this changes with different language versions of windows. Is ashDisp.exe in that location ?

I don’t use Vista so I’m not sure if this is the same for Vista:
Make a link to ashdisp.exe in your startup folder.
Or Add the path to ashDisp.exe into a value named avast! in the Windows Registry key HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run