when i run eraser 5.7 will it erase my operating system?

No. It will only delete anything in free space and cluster tips of files and any files that you choose to delete manually (either by using the ON Demand feature of Eraser or by right clicking on the name of the file you wish to delete and choosing Eraser from the context menu).

DBAN is the product that wipes everything on your hard drive, including the Operating System. DBAN is bundled with Eraser, so just leave that particlular program alone.

On the other hand if you want to wipe everything including the Operating System then it's DBAN you need to use. It can be copied to a floppy disk or burned to a CD and is ran as the computer boots up, before Windows loads.