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I've already reinstalled siduction.
This time along with another distribution to be on the safe side...
So you have worked much, the problem is not understood, not solved but only circumvented.
Installing an other OS doesn't help to understand either.
Hint, hint, hint: If you d-u and your machine boots but fails to switch to X it is most likely kernel/grafics related. Seldomly it may be related to the loginmanager (kdm, gdm, slim).
This is what you should be aware of and then provide the neccesary info (which grafics card, which driver, new kernel?) What exactly can you still see (blinking cursor left upper corne on blank black screen?). Can you still switch to konsole (ctrl-alt-F2).
This would be usefull info and a first step for walking a flat learning curve.