Mac OS won't install from CD, but Windows will

I had to replace the hard drive in my 2010 21.5" iMac. When I went to install the OS, I rebooted into the OS selection screen (using option). The Mac OS X install disc was there (but my external HD, where I back up wasn't), so I selected it.

It starts by showing the Apple logo, as expected, but after a while, the logo changed to a "cannot proceed" icon (circle with a slash). Thinking I installed the drive incorrectly, I tried my old WinXP CD.

Long story short, XP is installing right now. I used my Mac CD trying to repair the install just before the HD failed, so I know it's readable.