fixed it :D !!
there is also a program for "professionally messing" with your MBR: super grub disc. unfortunatelly it didn't work for me in sense of installing a working MBR, but it did help to access both OS -which i couldn't otherwise!
anyhow... ...i'm not quite sure if it was/is the most elegant way to do it, but this is how i succeeded:
1) i started installing winXP a second time and broke off the install-process (still don't know where the install-files are and how to remove them). after this i only had the win-boot-loader (with my working win and the installation-process).
2) next i used the edgy-eft install disc to get into the knoppix. there i went into the terminal:
a) Type "grub" which makes a GRUB prompt appear.
b) Type "find /boot/grub/stage1". You'll get a response like "(hd0)" or in my case "(hd0,3)". Use whatever your computer spits out for the following lines.
c) Type "root (hd0,3)".
d) then "setup (hd0)" to write GRUB into the MBR (master boot record). [another instructions used "setup (hd0,3)" this writes it to your linux root partition.]
e) type "quit".
f) restart.
now basically you're done. you do have a fresh GRUB which might be working.
in my case i still did have to change the entries of the GRUB a bit (you can do this immediatelly at boot up (directly in GRUB). but this is only temporary (good for experimenting :)).
i did have to change the WIN entry to
root **(hd0,0)**
and the ubuntu-entry
title Ubuntu, kernel 2.6.17-10-*whatever*
root **(hd0,1)**
kernel /boot/vmlinuz-2.6.17-10-*whatever* root=**correct_hdd_name** ro quiet splash
initrd /boot/initrd.img-2.6.17-10-*whatever*
savedefault
boot
to the appropriate disc...
the only thing that is not "clean" is, that i do get the win-boot-loader after choosing win (in GRUB). in win-boot-loader i then have to choose between the working win and the install-process *(i hope to get rid of the install thingie somehow)*. if you know HOW, please tell me :)
well... i have no clue if you understood this (it's my first "how-to"...i'm quite a newbie) if something is unclear, feel free to ask (write a message).