1. Please open "WebShield.ini" by notepad.exe.
2000 / XP:
C:\Documents and Settings\All Users\Application Data\Alwil Software\Avast5
Vista / 7:
C:\ProgramData\Alwil Software\Avast5
2. Put these phrases at "[WebScanner]" section:
OptinProcess=sleipnir.exe (or your browser's file name)
3. Save it. Avast's self-defense may alert you, so allow it.
P.S.
This link shows same procedure in Japanese: if you are Japanese, this can be more easy to follow.
http://pc11.2ch.net/test/read.cgi/sec/1276671434/4