unfortunately I don't know how to do it. Is it by editing the registry?
No, the C:\Program Files\Alwil Software\Avast4\Data\avast4.ini file.
But you can download avast External Control on the link 'Control' in my signature.
Use the proper option to tweak your installation.
Is the problem due to absence of Internet Explorer? During installation, there was a message that the program requires IE 4 or later.
I think not, IE is only needed to display help files and other HTML issues not related to your problem.