Restoring 8.6 On My Imac, Why do you need to hold down C? Options

[gorillacake]

O.K. my imac is a mess and I want to completely erase my harddrive and reinstall 8.6 from scratch.
I know I need to use my software restore disc but everytime I try to start the computer (disk is inserted) and hold down the c key I get a black screen. What's the difference between just clicking on the restore cd Icon on my desktop and actually restarting it and holding the c key?

[gorillacake]

O.k. for the benefit of any newbie who has followed this post I will follow through and talk about my sollution. I realized that since I had the 9.2 install disks there was no reason to install 8.6 first. So I inserted 9.2 and restarted the computer holding c so as to start up from the disk. I could then initialize (erase everything) the hard drive and do a fresh install of 9.2. I then went to apple.com (after setting up my internet connection), downloaded the 4.8.1(I think) firmware installer and followed the detailed directions on how to do this. Make sure you read these because you have to hold in the programmer button on the side of your imac while starting it up to install the firmware. Your computer will then start up in 9.2 all shiny and happy. There was a big difference in the graphics resolution even from when 10.2 wasn't installed. I then did the easy install of 10.2 and my mac was happy! I guess the important thing I learned was that in order to initialize a hard drive you have to start up from an OS install disk. Hope this helps someone.