The OS for a Raspberry Pi won’t work directly because it’s compiled specifically for that computer. BUT, the Raspberry Pi OS is based on Debian Linux, which is a free operating system that’s available to download from http://www.debian.org
You basically download the image that they make available, burn it to a DVD, reboot your computer, and tell it to use the DVD drive to load from. After a bit of a wait, there will be instructions that tell you what to do (there are also decent instructions on the Debian website).
I’d like to expand a tad on Tim’s excellent answer (anything to do with Linux is excellent 🙂 ):
There are other distros (operating systems based on the Linux kernel) that provide “LiveCDs” which basically are DVDs (or in some cases still CDs) that you boot the computer from, it will let you have a play around with that operating system without doing anything to your hard drive. (It’d be good to bear in mind that running off a LiveCD is always slower than running off the main drive).
So if you find a distro that suits your taste, you can then tell the LiveCD to install itself onto your computer (of course after backing up any files you want to keep 🙂 )
I’ve tried quite a number of distros, and I seem to have settled for LinuxMint KDE (which is derived from Debian too), and ArchLinux:
ArchLinux has a very steep learning curve to start with, but you get total control over your system with Arch. I would recommend that anyone coming over to Linux from Windows to try the other distros first, and over time, if they want to, moving to ArchLinux will give you far greater understanding of how operating systems work and knowledge to fix things should anything go wrong 🙂 https://wiki.archlinux.org/index.php/Arch_Linux
Just to add a bit here. In general pc are based on a architecture called x86. This family of processors date from 1978 and is what is used for all Pentium, Core Duo, i3/i5/i7 and so on.
So you can choose most OS who support those processor and Linux is one, you could also install another version of Windows, it all depends what you want to do with the machine. Generally Linux is great as much lighter than windows and free. If it is your first time wanting to get into the Linux world I would suggest you to use Ubuntu which is very intuitive and has an interfact which is close to Windows.