When i try to run avast after the memory scan a window come’s up saying:
Unknown error.
Skin is not complete. Look at the following descreption: Skin is not loaded properly.
What should I do?
Can you try to repair your installation?
Go to Control Panel > Add/Remove programs > avast! antivirus > Remove. Then choose Repair function in the popup window (Repair). You must be connected to the internet while repairing.
If this does not help, can you uninstall / boot / install / boot again?
If the error persists, can you ask for further help?
i tried repairing it but it didn’t work.
Welcome to the forums, qaz79. 
And did you try the second part of Tech’s advice? ???
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 \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