The step-by-step process varies based on whether it is a retail copy of Vista you purchased or if it came preinstalled, and, in the case of the latter, by which manufacturer. The good news is that the Vista installation process is much simpler than that of its predecessors. If you insert the DVD and reboot you should automatically be prompted to boot from CD/DVD by pressing any key...do so. (If you're not prompted you'll have to edit the BIOS settings first...just let us know the computer's make/model.) If it's an OEM/preinstalled copy it will automatically reformat the hard drive, whereas if it's a retail copy you will reach a screen that displays a list of partitions (logical divisions of your hard drive) along with a Format button. (Picture of what to look for if it's a retail copy.) Just remember to backup any/all important files first...they won't be recoverable after you reformat and reinstall Vista.

Hope this helps,
John