Graham,
I've had every Windows version of BIAB since 2006 and I have never seen a file called soloist.dmg with any installation.
I had a read through the below link and PG Music do indeed provide .dmg files for Mac installations.
http://www.pgmusic.com/salesfaq2.htmWith the above in mind, it seems likely that if you picked up soloist.dmg from PG Music, you might have gotten a Mac file rather than a Windows file. If this is the case, contact PG Music. They'll sort it out for you. (There are a number of different contact option listed under the "Support" link at the top of the page.)
Regards,
Noel