Q: Unable to Install Leopard in my late 2006 Imac.

Have you read the install instructions that came with the disc? All you should need to do is put the DVD in the SuperDrive, when it's seen double click the DVD and the Install instructions will begin. Where did you get the Leopard disc? If it's not the retail version it will not work on your machine, if this disc is borrowed from another Mac that had Leopard it will not work because those discs are machine specific.

Information courtesy of TexasMacMan;

Mac OS X 10.5 Leopard installation system requirements

http://support.apple.com/kb/TA24950

Leopard is no longer available at the Apple Store but may be available by calling Apple Phone Sales @ 1-800-MY-APPLE (1-800-692-7753).

If you can't obtain a retail install DVD from Apple, look on eBay or Google the installer part numbers to possibly find at an on-line store. Here's what to look for:

MB427Z/A  Leopard 10.5.1 install DVD

MB576Z/A  Leopard 10.5.4 install DVD

MB021Z/A  Leopard 10.5.6 install DVD (single user)

MB022Z/A  Leopard 10.5.6 install DVD (5-user family pack)

Installing Mac OS X 10.5 Leopard

http://support.apple.com/kb/HT1544

Mac OS X 10.5 Leopard Installation and Setup Guide

http://manuals.info.apple.com/en/leopard_install-setup.pdf

After you install the base 10.5, download & install the 10.5.8 combo update at http://support.apple.com/downloads/Mac_OS_X_10_5_8_Combo_Update

if your disc is not a black one with a big X on it, and one of the above serial nos. it won't work.

No grey disc or any with "upgrade" or "drop in" on the label will work.

Can you post the model identifier of your Mac?

If it's 2006, it should be an Intel Core Duo of at least 1.83 GHz, so should have no difficulty with Leopard provided you have sufficient RAM.