No they are not. But last night I managed to fix the problem
If somebody happens to have the same problem, here is what I did:
As described above I replaced the system hard drive with a new one and reinstalled Windows because it didn't boot at all.
By unplugging all the raid discs and reconnecting them I finally found out that one drive seemed to be the cause of all problems. So I went to the store, got a new one and replaced it. Now the system wouldn't boot anymore again. So I disconnected the new drive and booted with three of the four raid drives.
When Windows was up I connected the last drive and set it as replacement in the Intel Raid software thing.
So the raid mounted and I backed up all data over night onto an external disc.
I still had to reinstall Windows because somehow my system disc wasn't recognized as bootable anymore but now it works smoothly again.
Thanks for your help!