There is a number of apps allowing you to virtualize an operating system, within another operating system. I found VirtualBox quite appealing, as it is free, has great support from the community, is extremely easy to use and covers a great number of OS's.
I successfully virtualized:
~Windows (98 -- which is still a crappy OS that takes alot of time to install and has weird processor handling -- XP and 7)
~Linux (Ubuntu, Red Hat its kid CentOS and Suse)
Before you atempt to virtualize MacOS, be sure your processor supports Virtualization, that is Intel-VT or AMD-V (depends on your processor brand).
You can check it with this (http://www.grc.com/securable.htm) tool made by Gibson Research Corp (Hack the Gibson! :-) )
*Note, that MacOS will not run as it is supposed to. Expect random crashes, stuff not working, etc.
I will write an extensive guide with pictures on VirtualBox in a new post.
You can get VirtualBox HERE.