Without installing anything over it? Download http://www.dban.org/download,
burn the CD from that file, and follow the instructions.
If you want to format and re-install Windows, make sure you have a OS disk to use, boot off that. During the first few parts of the setup process it will ask you where to install it. Select the existing partition, and delete it. Next step it will ask you where to install it again, select the blank drive space, it will ask you to format it.
Keep in mind that formatting, and especially the wipe CD will delete all of your data on the drive.