OK, check the following key:
HKEY_CURRENT_USER\Software\ALWIL Software\Avast\4.0
Is there a value called CurrentSkin there?
If not, create it (as a string value) and set it to Silver Panel.asws, or whatever your skin filename is.
Correction: it would work if the value wasn't present - but if the value CurrentSkin is present, but contains an invalid filename, it really behaves as you described. So, delete the value and avast! should recreate it when it's restarted.