So I am done. Unfortunatley the result is near worst case.

On my Notebook I had already a dual boot installation (GRUB-> XP Home + SUSE 9..2).
Knoppix install killed grub. Now LILO comes up after booting and offers me 3 Knoppix variants and my WInXP.
Unfortunately:
- SUSE 9.2 is not referenced in LILO
- the 3 Knoppix variants do not boot ("Kernel Panic ..." during boot routine)
- without the USB drive connected the PC does not boot at all.

I assumed that the MBR would only be changed on the USB disk, not on the PC´s main disk.
So first I have to restore the PC (to boot without USB disk and to find XP and SUSE).
I think I will have to dig into MBR / Bootloader stuff now... ;-(
Maybe I open another thread here in the forum?

Thanks anyway!
Cheers Thomas