Yes it's possible to edit the windows registry from within Knoppix using a tool called "chntpw". I go over this in detail in Knoppix Hacks, but here's the gist of it:
Download the chntpw deb from http://packages.debian.org/unstable/admin/chntpw
Extract the chntpw binary by turning the deb into a tarball:
Then untar it. Make sure the windows partition is mounted read/write (if it's NTFS you'll need to make sure you have already set up captive-ntfs). Then you can use the chntpw utility to access a specific hive (usually you'll want something like the SAM or SOFTWARE hive under winnt\system32\config):
alien --to-tgz chntpw_0.99.2-1_i386.deb
As you can see I just left the chntpw utility in my home directory. The chntpw tool has an integrated help you can use to get a handle on how to use it, but basically it's like navigating a filesystem. Once you are finished editing the registry, sync, and unmount your partition to make sure the changes stuck (extra important if you are using captive-ntfs). Also make a backup copy of the registry hive you are editing just in case.