Welcome lazlina
You are right - Linux is free!

And there are a lot of choices. but i think there are about 3 distros for you to choose from:
Linux Mint Xfce

You can click
here to download it
Zorin will let you change the layout to Win XP or Win 7 with the click of a button. So it will look exactly like you are used to.

You can download it
hereMageiaYou can download it
hereI have provided links to the 32 bit versions - If you are not sure if you need 32 bit or 64 bit versions - then you can simply click on the links I have provided here.
If you need the 64 bit versions just go to the webpage and download it from there.
Other Linux version that you can use are:
Manjaro - it still is a fairly new distro. and has the same installer as Mint. It will probably be the fastest distro and have the newest software. It also comes with steam. And it looks good but not exactly like windows:
You can download it
hereIf you have a Windows system today then you can use
cdburner (it is free) to burn the downloaded iso files to the DVD.
After that you simply start your computer with the DVD in it and you can install Linux to your new hard drive from there.
Let us know what you picked and if you are stuck feel free to ask - remember:
There are no bad questions - only bad answers
(Moving this to Linux education)