ASF stands for "Advanced Streaming format" and is a proprietary codec courtesy of Microsoft Corp.
It is used to compress Audio and Video data into one single file, so to say.
My best bet is that Windows Media Player is able to handle it - to view Windows Media within Quicktime you go flip4mac, like the other here said already.
To be able to convert this from Quicktime Player and export into other formats ( for use in imovie ) you will likely need to have a Quicktime Pro License, which will unlock those features in Quicktime.
Another attempt to handle that file would be the free VLC - Player for Mac OS X ( www.vlc.org ) which can play amost any file available,even exotic ones including encoded and encrypted DVD, which no other free player would allow.
Good Luck