If you have the windows install disc, you can insert it, restart your computer and press any key when it tells you to. This will boot from the DVD and enter the Windows Setup. From there, it will show you a list of partitions on your hard drive before you install windows. Choose the partition on which Vista is installed and click "Delete" under advanced options. The partition will be formatted. If you want to install a fresh copy of Windows Vista, you can do so now. If not, exit windows setup and install whatever OS you want to use.