Thanks Maxell, that looks like a good site.The link you gave me was to this Mobos sister board the P4S800-MX_SE but the P4S809-MX is bound to be there somewhere.
Well, you wouldn't believe it, I emailed ASUS a few hours ago about this problem and they got back to me as I was typing this. The "Lite" version of the Install program is on their support site not the "Full" version. They have given me a link to FTP it. Why the "Full" version is not there who knows.
Anyway, thanks to all who replied!