When are you getting this error ?
Try a repair of avast. Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button and scroll down to Repair, click next and follow.
If that doesn't work, try (this is related to XP check the path to files on your OS if different):
Windows, Run and type:
C:\WINDOWS\system32\rundll32.exe "C:\Program Files\Alwil Software\Avast4\Setup\setiface.dll",RunSetup
What do you get?