also....
I believe you are saying that you have no OS on your iMac anymore so you would not be able to go to the apple menu and click more info..
Look at the base of the iMac, the part that actually sits against the table and tell us the model number and other numbers on the bottom. One of those numbers should match the number on your CD. Usually it is something like A1213, or possible something similar to 8,1 or 10,1 depending on the year etc..
do that and post back, you are probably using an upgrade disk.
Also, are the disks grey, or do they have an OSX logo on them like the blue tiger or purple Leopard splash screen?