The message says "The partition might contain recovery files, system files, or important software from your computer manufacturer. If you delete this partition, any data stored on it will be lost."
Click the OK button.
Important: As I spelled out in the last step, please be aware that all the data stored on that drive will be lost. If you have not backed up everything you want to keep, click Cancel, end the Windows 7 clean install process, restart your computer to boot back into whatever operating system you have installed, and backup everything you want to keep.
To be clear: This is the point of no return! There's no reason to be scared, I just want it to be very clear that you can't undo the deletion of the drive you selected after you click this OK button.