Honestly, the easiest way to handle this is just to grab a Linux Mint live USB. Here’s how I’d go about it:
1. Download Linux
http://www.linuxmint.com/download.php2. Once that's done, you'll need to burn that ISO file onto a blank DVD—or, better yet, just flash it to a thumb drive.
3. Head into your BIOS settings and make sure your CD/DVD drive (or USB drive) is set as the primary boot device.
4. Boot into Linux, and that should be pretty much it—no full installation required. You can just plug in an external hard drive or a USB stick, move all your important files over there, and then you're free to format the main drive and install a fresh version of Windows.
p.s. I'm assuming you're comfortable with the actual Windows installation process, right?