I keep receiving "Engine Unavailable reinstall or contact support".

i have installed the avast on a intel mac mini with OS X 10.5, at the first time installed, it worked well, but i made some problems so the system needed to be re install. After reinstall the system, i install avast again, however, this time, when it loads, I keep receiving a error “Engine Unavailable reinstall or contact support”.
i try to reinstall, but no use, how can i solve it, thank you!

This is a known bug: See this topic:
http://forum.avast.com/index.php?topic=32745.msg277495#msg277495