put the DVD in the drive and restart your computer, it should boot from the CD and give you an option to reinstall.
as far as running two operating systems, its always better to have each one on a separate drive. in this case I would get a blank External drive and load Mint to that. then when you want to use mint, just go into the BIOS and tell the computer to boot from that drive first. This way whenever you have the external plugged in, it will boot to mint first.