Originally Posted by walthi
Can I solve the problem with a new or a blank Harddisk?
That would be an extreme solution.....
Try booting up with any Linux Live CD (Your Ubuntu disk is probably one), open a terminal, and enter "fdisk -l" or "sudo fdisk -l". Post the results here.
Depending on the setup, you might be able to install GRUB to the mbr. (Tho previous post talks about getting the Windows loader in the mbr.)