It's going to be a tad difficult without knowing a little more about your setup but I'll try and be as general as I can. First, you need to get yourself a "Live" type distro on a bootable CD that has the "mount" and "chroot" commands. I don't know about Knoppix but I know the Gentoo stage 1 LiveCDs have those commands as they're essential in installing Gentoo. Once you have the CD and have booted into whatever environment it sets you up in you're going to have to mount the Linux partitions on your hard disk. Before you can do that though you're more than likely going to have to create a "container" directory for the mounted Linux filesystem.
To do that, launch a shell (it's important this all be done as root btw, make sure whatever LiveCD you use gives you this capability. The Gentoo CDs do but I'm not sure about anything else), change to the /mnt directory and then create your container directory (I usually name it according to what distro gets mounted there, a habit from installing Gentoo, but for simplicity I'm just going to call it linux) using the mkdir command. To summarise:
Now, to mount the partitions type:
mount /dev/hdxy /mnt/linux/mountpoint
where x represents the drive the partition is located on, y represents the number of the partition and mountpoint is the directory that partition normally gets mounted into (such as /home, /usr etc. / just gets mounted into /mnt/linux and should be mounted first).
Once you have your partitions mounted, use the chroot command to "enter" your Linux installation:
chroot /mnt/linux /bin/bash
and from there you can run the commands to restore your bootloader. Once you've finished, type exit to exit the chroot shell and then unmount your partitions (from last to first making sure you're not in any of the directories your trying to unmount) using the umount command on their mount points. To unmount a partition you use the umount command. So, for example to unmount the /mnt/linux directory you type:
Once all that's done, you can shutdown the LiveCD and hopefully your bootloader will be restored allowing you to boot into your existing Linux installation (just remember to remove the LiveCD from your drive and/or set the BIOS up to boot from your hard drive first!).