I'm pretty sure that Eraser can't write directly to a CD, so you will have to go to the website for DBAN (Darik's Boot And Nuke) and download the ISO from there (dban.sourceforge.net)
You will need to use your CD writer software to burn the ISO disk image to CD. You might need to check your manual on how to burn ISO files. Then you can boot up with the CD and completely wipe your hard drive(s). But be aware that DBAN wipes out everything, including any operating system and formatting.