Read the instructions,
To erase your entire hard disk:
Eraser uses the 'Darik's Boot and Nuke' disk option to erase all your hard drives.
To use this option:
1. Place a floppy (or CD) in your a: drive.
2. Click on the Start button and then choose Programs->Eraser->Create Boot Nuke Disk.
3. Ensure 'Writing on Floppy' is ticked. Tick 'Formatting' if your floppy needs to be formatted.
4. Click OK. Result: Your Boot Nuke Disk has now been created.
5. Boot up your PC with the floppy still remaining in your a: drive.
Remember: If you wish to erase the Hard Disk of another PC, then bring this Floppy to the PC, insert it into the a: drive and Boot it up