Your Windows installer is not recognized by boot camp as a valid (bootable) installer disk.
Try an alternate method - here's two slightly different ones.
Quit Boot Camp, if it's running. Insert your Windows installer CD, then restart your Mac. When the screen goes black, hold the letter C, which will force your Mac to attempt booting to your Windows installer.
What do you do if you simply boot back to Mac OS? You'll quickly tell that the grey Apple comes on the screen, which won't happen when the Windows installer boots.
If you get the grey Apple, press and hold the power button for 5 seconds, which should shut your Mac off.
Then, press and release the power button, and immediately hold the Option key. If the Windows installer is bootable, you will see it in the boot-picker screen that will appear.
If you don't see an icon for the Windows installer, then press the eject key on your Mac keyboard. Any disk should eject. Wait for 10 seconds, and reinsert the CD. If the Windows installer icon does NOT appear, then I suspect that your CD is no good.
Make another copy of your Windows installer. Best bet is to use the .iso that you downloaded from Microsoft. If you don't have that .iso now, you should be able to download it again. If you go to the Microsoft store, you should be able to find out how to do that. Or, use the original installer CD, as you should still have that, eh? If you can successfully burn a copy, that new copy may boot your Mac.
Finally, Boot Camp requires a full Windows install, and not a Windows Update install. Are you certain that your installer is a full install?