I had the same problem and found that it has something to do with the disk previously having had a GUID partition table but not completely wiping it out before changing to an MBR partition table. This results in an MBR partition table with corrupt GPT signatures, which gparted chokes on.
Before doing anything it's best to backup the current MBR, in my case Windows is installed on /dev/sda:
sudo dd if=/dev/sda of=sda.mbr bs=512 count=1
To clear the GPT signatures you'll need gdisk (apt-get install gdisk):
sudo gdisk /dev/sda
Switch to expert mode:
To "Zap" GPT enter:
The selection for the first question doesn't matter (MBR or GPT), be sure not to blank out the MBR.
Once you've done this you should be able to relaunch the installer and it will detect the partitions along with windows for dual-booting.