Booting to a Windows dvd will give you the option to format your hard drive during the install process. If you don't want to install Windows you can just remove the disk and shut down the machine after you have formatted the drive.
This will delete the data on the drive so have anything you want from it backed up. It's not a secure erase, but it would take some effort to get the data back after you have formatted it.
You could try downloading or obtaining something like Puppy linux, which runs from the cd without needing to install
It includes GParted, which will let you delete the partition, create a new one, and format it in several file systems