With Firefox, if you download (to save), the file comes in OK - but if you try to open it ("with") immediately through Firefox, it pukes up the same error.
Incidentally, on the successful save through Firefox - I immediately scanned the file and it's clean.
This isn't a virus problem, it's a compression format error.
They got too excited about sharing it free and didn't care to test it out across platforms.
Not surprised.