You cannot install or run an OS X version earlier than the version that was loaded onto the Mac when it was released.
If you have old PowerPC software, it can still run on any Intel mac built earlier than mid 2009 running 10.6.8 - Snow Leopard with the Rosetta application installed.
Rosetta is a dynamic binary translator that allows programs written for PowerPC's to be translated into Intel instructions for use on Intel Macs running 10.4.4 to 10.6.8
The last OS X version to support PowerPC's was Leopard 10.5.8.
Applications written for PowerPC's became obsolete for new Macs on 20 July 2011 when OS X Lion 10.7 was announced and support for Rosetta was dropped.
OS X allows for Microsoft Windows to be installed and run via the boot camp assistant, which creates a Windows partition on the Macintosh HD. You can choose which operating system to boot at startup.
One of the benefits of OS X over Windows is the ease of upgrading OS X versions. Programs do not need to be re-installed and settings remain unchanged after an upgrade. You can install OS X 10.7, 10.8 or even 10.9 directly from version 10.6.8 in as little as 30 minutes depending on the machine.
OS X is machine independent. Providing the OS X version can be run on another machine, you can just swap the hard drive over without any modification to OS X.
If you're old mac breaks down but the hard drive is ok, a new mac can transfer all your data, settings, profiles and even programs to the new mac using the fully automated migration assistant tool.
Intel OS X versions with release dates and minimum system requirements:
10.4.4 Tiger. Released - 10 January 2006
First Intel Core Macs came with OS X Tiger. Tiger is capable of running on both Power PC and the first Intel Core Macs.
10.5 through 10.5.8 Leopard Released - 26 October 2007 Hardware requirements for Leopard include a minimum of 512MB of system memory.
10.6 through 10.6.8 Snow Leopard Released - 28 August 2009 Hardware requirements for Snow Leopard included a minimum of 1GB system memory and Intel processor. 10.6 is a 64 bit operating system
10.7 through 10.7.5 Lion Released - 20 July 2011 Hardware requirements for Lion included a minimum of 2GB system memory and Intel Core 2 Duo processor
10.8 through 10.8.5 Mountain Lion Released - 25 July 2012 Hardware requirements for Mountain Lion include a minimum of 2GB system memory and the following Macs:
iMac mid 2007 on
MacBook Pro 2007 on
MacBook Air late 2008 on
Mac Mini 2009 on
Mac Pro 2008 on
Aluminum and Polycarbonate 2009 MacBook on
10.9 through 10.9.4 Mavericks Released - 22 October 2013 Hardware requirements same as Mountain Lion
10.10 Yosemite Release - October 2014 Hardware requirements same as Mountain Lion
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