Why is it that you feel you need to run a scan from safe mode ?
Firstly you shouldn't need to run a scan in safe mode, unless you have an older OS, win9x, winME, newer OSes win2k, winXP, Vista, etc. offer a better option in the form of a boot-time scan carried out before windows starts.
If you have XP, vista32bit or Win2k, you could enable a boot time scan. Right click the avast icon, select Start avast! Antivirus, a memory scan will take place followed by the opening of the Simple User Interface, Menu, 'Schedule boot-time scan...' Or see http://www.digitalred.com/avast-boot-time.php
If in safe mode avast isn't started, clicking the desktop avast shortcut icon should just do the memory scan followed by opening the Simple User Interface, yes you would get the help notes. I don't know if having safe mode with networking would make any difference unless avast is installed in a different location to the default (even then I doubt it), but lets not cross that bridge until you answer the above question, why you need to do this, it is the root of issue I think.
There is a possibility that your installation is not correct, why the simple user interface isn't working. From a normal windows boot - Try a repair of avast. Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button and scroll down to Repair, click next and follow.
Problems like this can come from many things but one common one is another AV or remnants of another one still on your system.
Have (or did) you another AV installed in this system, if so what was it and how did you get rid of it ?