What type of harddisk is it? If its SATA you will need to load the sata.i instead of bare.i.
Also remember that sata disks usually are named sda, sdb and so on. Normal disks: hda, hdb...
You can just do a fdisk on your harddrive before trying to install Slackware, and perhaps also a fdisk /mbr
Then you will know for sure that you are starting on clean system.