Unable to remove OS

I am running Parallels 6 on a MacBook Pro (Snow Leopard OS X 10.6.5; 2.66 GHz Intel Core 2 Duo; 4 GB MHz DDR3 RAM). Then I installed Windows 7 Home Premium as the Windows program.

Due to some problems I would like to remove Windows 7 from Parallels 6. However, what I understand to be the normal removal procedure will not work. In Parallels Desktop > File > Remove, the "Remove" option is grayed out (please see screen shot below).

Does anyone know how to correct this, or another way to remove the OS from Parallels?

Thanks in advance for any advice, comments, suggestions, etc.