In addition to all the requirements for running Knoppix from the CD, you need the following to install it onto your hard drive (these are bare minimum requirements; as with most things involving computers, more is always better):
2GB or more of hard drive space to install the primary operating system
128MB of swap space (if you have less than 512MB of physical RAM).
You can set up partitions for these prior to starting the installation routine or you can wait until the install process and use QTParted.