It's relatively simple. Download your OS of choice onto a USB drive (or CD), put it into the computer, and then go into your BIOS and set that as your boot drive. Restart you computer, and go through the installation steps. You will then boot into your OS of choice. Restart again, go into BIOS, and select the hard drive that you installed the OS on as the boot drive. Restart and you should boot strait into your OS. Note though, it has been a while since I last had to do this, and I may have missed a step. I would wait for more replies before taking action. If an OS is already installed on the hard drive, then simply go into BIOS and set that as the boot drive.