As of 04/20/2014, and with Win 7 Ulltimate 64-bit, I'm finding my Avast! log files here:
C:\Users\All Users\AVAST Software\Avast\log
This is good coding practice because Microsoft discourages using the installation folders for installed programs to store such data. Instead, locations like the one above are preferred.
Of course, MS is the first to break their own rules. ;-) MS SQL Server, for examples, stores its database files under its installed folder.