upgrading f15 -> f16

Hi,
I have 500 gb hard disk initially loaded with windows 7. Later, i created a partition and installed fedora15. Now, i tried upgrading it using preupgrade. This successfully downloaded all the required packages and asked me to reboot the system. Upon rebooting I choose the option upgrade after which i got a message saying "could not find any empty partition".....

Can anyone help me out?

Thanks