As a workaround and a test, you could disable skinning in avast and see if that should solves the problem:
To do that, open the file <avast>\data\avast4.ini, look up the [UserInterface] section, and add the following entry:
StartWithSkin=0
Save the file, boot and try to start avast again.
If this does not work, try running C:\Program Files\Alwil Software\Avast4\ashSimp2.exe