When you reboot, press whichever key gets you into the BIOS (usually Del or F1 or F2), and look for the boot options or boot order. Just make sure it's set to boot from CD or DVD first, then HDD second, & the rest is easy. If you're SURE you want to delete the old OS completely, & start from scratch, just delete that partition, when it asks you where you want to install. Vista & 7 are pretty good about things running smoothly from there, as far as cluster size & all that good stuff. It'll always have a small partition marked "reserved", that's just what Windows needs for elbow room, during the install.