Ubiquity doesn't see any partitions on the Disk

I want to install Xubuntu 12.10 on a system which has already Windows 7 on it. However, the Ubiquity installation program does not recognize any partitions on the system and shows it to me as empty. But fdisk and /dev list the partitions to me, and I can mount them manually.

From this question I tried the provided solutions, but neither the dmraid -rE command nor running the ubiquity via sudo helped here. According to the last recommendation, I used boot-repair to sum up the trouble, and the paste can be found here. Obviously, os-prober does see the Windows installation, just ubiquity doesn't.

I'd be very thankful for a quick help, I need the dual-boot system for work.

After about three hours, I found out that the Windows 7 installer had originally written the partitions bigger than the harddisk claimed to be. Thus the libparted of Ubiquity thought the partition table was corrupted.

The solution was quite simple: Just shrink the windows partition to the desired size and leave the rest of the disk empty. fdisk can read the oversized partition table just fine.

Afterwards use Ubiquity like normal to create the necessary partitions on the empty space - libparted will make sure they're the size of the harddisk.