Ubuntu Desktop GNU/Linux 14.04.3 Trusty Tahr LTS x64
Long term support version of the famous free and open source OS.
64bit version.

Update 14.04.3 – 2015.08.06

System requirements:
- x64 capable CPU
- 700+ MHz processor
- 512+ MB RAM
- 5+ GB of hard drive space
- VGA capable of 1024x768+ screen resolution (from my experience, worked also on 1024x600)
- Either a CD/DVD drive or a USB port for the installer media
- (Internet access is helpful)

Installation guide: 
- Burn the ISO to DVD or use the Pendrive Linux program, to create a startup flash drive (Startup Disk Creator - if you use Ubuntu).
- In the BIOS settings of your PC, select DVD drive / USB flash drive (respectively) as your First Boot Device.
- Follow the instructions of Ubuntu installer.
- When the PC restarts during installation - log on to BIOS (DEL key, or F2 or other button), and select your Hard Drive as the First Boot Device.
- The installation will continue and you will log on to your Ubuntu desktop.

Notes:
- Ubuntu LTS version will be supported for 5 years (until April 2019).
- Even though you can see the name 'amd64', the OS will work on any x86_64 CPU, both Intel or AMD. 

Enjoy!