|Thank you for the answer!|
I thought that I could clean the harddrive in DOS and then try installing again with my recovery disc but it seems not to be a good idea.
When I now start the recovery disc this message shows (translated by me):
"It wasn't possible to start Windows because of a configuration fault on the harddisk.
It wasn't possible to read from the specifyed start disk. Check the start path and the disk.
Check in the documentation for Windows about the configuration of the harddrive and the reference manual to the hardware for further information."
The only documentation I have tells me to start the recovery disc which doesn't work - the computer locks with the message above.
Perhaps the only way is to send the computer to HP-support, but that will coast me almost $300 :-(