well i'm not an expert but my first guess would be the setup of your hard drive. not sure if cfdisk, or slack for that matter, likes sata drives. second, the fact that you said your bios is outdated, i would really try to find an update for it.
also if you already made the partitions with partition magic you can skip the cfdisk part of the setup and just run setup after booting with the slack cd. it will see the linux partitions and ask you where you want your swap partition and then where you want to mount any other partition
hope that helps