On the machine I'm currently using. XP Pro SP3 (I think there are differences fro XP Home), the logs are at
"C:\Documents and Settings\All Users\Application Data\AVAST Software\Avast\log"
EDIT: Checked on XP Home, a small difference "C:\Documents and Settings\All Users\Application Data\Alwil Software\Avast5\log"
As the others have said, probably because it was upgraded from 5 to 6.