The problem with deleting files off an old computer is that you cant do it from within Windows because Windows does not fully delete a file and old files can easily be recovered.
So the only way is to "boot" a computer from a diskette or CD and then run the program from there. The diskette or CD then has exclusive access to the hard disk and can write rubbish over it thousands of time to erase anything that was on the hard disk.
The program that most people suggest is called DBAN (Dariks Boot and Nuke).
http://www.dban.org/
You need to download the file and then make a bootable diskette or CD.
You then need to boot the computer from this diskette or CD, then run the proigram.
You say you are a novice so you may need some help, but I did it a year or so back and dont remember it being that difficult.
Good luck.