Just to make sure -- have you verified that the Slack 9.1 install disk is OK? Assuming you can boot off CD-ROM (and you CAN), I would be surprised that the Slackware install disk wouldn't work. How ...

I wanted to install Slackware. I downloaded, checked md5, burned to CD, and rebooted. Selected media, selected target partition, selected a full install, and then told it to go ahead and install.