Since apparently you still have a working Internet connection on another machine, you can download a copy of DBAN (free), burn it to disk. For the burning, use IMGBURN (free also) or the 'burn image' option of your favorite burning program.
Then on that Vista machine, boot from that disk and tell it to erase the hard disk. When it's ready the hard disk is totally empty and the machine totally unusable for anything else than installing another copy of an operating system or running a Linux Live disk or USB-stick.

Kees