There are two files you have to download for BISE1 from OTN:
In Firefox, the file extensions are preserved the way they are, so that the two files are distinct when downloaded. No problem here - see the screenshot below.
But when trying to download it using IE, the extension is changed to .zip
IE for some reason decides that z01 should really map to zip, so that downloading both files results in one being overwritten.
So the solution, workaround, is to download them using the "Save As" option, manually specify the extensions to avoid this problem...
Wonder why this is happening why the way it is - this happened to me on two separate machines, and at least one other person from the field faced the same issue.