Abinn, boot ur computer with OS CD you have. for that go to bios set up and give 1st priority to boot ur computer from cd rom. then u can format ur  computer from there. But I think as you install (fresh copy not upgrade) ur OS it automatically format ur hard disk. Not sure about this but worth a try because the worst thing it could do is delete ur old file, but you are loosing your old files anyway.
good luck