Create a file with the .REG extension and put the following in it:

REGEDIT4

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Exchange\Client\Extensions]
"avast! 4"="4.0;c:\\program files\\alwil software\\avast4\\ashOutXt.dll;1;10000111111000"

Then quit Outlook, double click the file and start Outlook again. You should see a splash screen of the avast Outlook plugin and the scanning should start working…