Quote Originally Posted by jeeves2001
Can I remove an older version of Linux (Xandros) and replace it with a hard disk install of Knoppix without losing all the data on my hard disk?
Part of the Knoppix installation routine is formatting the / (root) partition. So, if you've only got one Linux partition (swap doesn't count here, obviously) then you'll lose whatever's on it during a Knoppix installation.

Possible solution: Shrink your existing root partition using cfdisk or QTParted (use Knoppix from the live CD if you need to), create another partition, and copy or move your data to the new partition.

P.S. Consider installing Kanotix instead of Knoppix. Unlike Knoppix, whose main purpose is to function as a live CD, Kanotix's main purpose is to get Debian Sid easily installed on your hard drive.