Wiped eMac HD refusing to accept install of any mac os?

When I power up my eMac, it goes on to a blue screen with a rotating cog, after the white screen with the Apple logo on it. I tried to reinstall from my Leopard install DVD, it loads up, goes to the language selection screen for installation but then it says that it cannot be installed on the computer. I know it can because I have done it before. I then tried to repair the disk and its permissions, but it said that it could not unmount "Macintosh HD". It can because I could unmount it manually from Disk Utility. I wiped it and tried again to reinstall but it still comes up with the same error. Any suggestions? Thanks in advance.