Hi Joy. It seems to me that you have a few options here.
The first option would require replacing the internal optical drive. If that's the route you desire to take, we would first need to identify your iMac so that we may provide you with the appropriate replacement guide.
Your second option is to purchase an external USB optical drive, such as the Apple USB SuperDrive. This would allow you to boot the Windows 7 install disc.
The third option is to install Windows 7 from a USB flash drive.
I hope these options are of some help.