Knoppix can be installed or i would recomend Debian Sarge. not as up to date but less trouble on the hard drive )Knoppix is designed to run from CD).
another advantage is that this forum can likely help you as Knoppix is based on Debian versions sarge and Sid.

Different versions of Debian are Debian stable (Woody) this is ROCK SOLID, and tested almost beyond reason.
Debian testing (sarge) not as tested but stable enough for 99% of situations (next stable release).
Then we have Debian unstable (Sid) whis is in constant change and upgrade... things might crash or not work well... if you are unlucky.

Id go with Debian or Slackware, they are NOT the easyest around but they work well when finaly configured, if you want easy i would go with Fedora, Suse or Mandrake. but they tend to be almost Windows, with that i mean somewhat dependant on wizards, menues... to get setup and configured, they might even delete changes made by you amnually...