You likely have too much free space - more than that setup knows how to properly deal with (numbers overflowing and wrapping around, things like that). 100+ gigs is an insane amount for W98, let alone a DOS program. Try filling up the drive with random garbage files until you get to, say, less than a gig free. You can clean it all up afterwards, but I'd actually recommend leaving it there and only free up as much as needed as you go, because you'll probably find this isn't the only piece of code with this kind of issue.