If you are already familiar with windows then I would suggest Linux Mint 14 Mate as an easy to install and use OS. If you are not that familiar with how to download and make your own DVD iso or thumb-drive, here is where you can buy a DVD that is ready to use.
Just insert and boot the computer. You may need to adjust the computer's boot order to boot from the cd/dvd player, this is usually done in the computer's bios advanced page. Linux mint will automatically install itself after first allowing you to take it for a test drive.
Linux Mint will auto-detect if you have windows or another OS installed and will allow you to choose for a dual-boot (meaning you can decide at boot-time which OS you want to run) or you can choose to use the entire hard disk drive for Linux Mint.
I suggest the Linux Mint Mate version as I have found this system seems to be the easiest for New Linux users to learn their way around in as the screens are similar to windows and the OS has everything you might need with a terrific forum and tutorials. Mint is a ubuntu based OS. Everything just seems to work, right out of the box, I think you'll like it.