No, it's not possible to run the Simple User Interface without any skin.
There is a low-res skin, that's used automatically for low monitor resolutions (or more likely for low number of colors (16), I'm not sure) - e.g. when Windows are running in Safe mode.
If you want to use it even for ordinary color resolutions, you can enable it - go to the avast4\Data\Skin folder, find the "low res.asws" file and rename it to anything else (e.g. "verysimple.asws"). Now, you will see the skin in the ordinary "Skin selection" dialog and you will be able to use it as any other skin.
But I warn you - it's really not a nice skin