Slackware 9.1 does require a hard disk. It is a standard (albeit superior) Linux distribution. The install disk is bootable, but it only boots a simple busybox-based system, with which you can install Slack to your hard drive.
There is an independent project called Slackware Live, which is a bootable Linux system similar to Knoppix, but based on Slack 9.0. To run this, you don't need to have partitions formatted for Linux, but you should certainly have a hard disk in your computer ;-)