eMachines Usually Have A Recovery Partition..
First, installing a new Windows also requires a legal Windows CD.. If you don't have either, then you'll need to purchase one from the manufacturer (eMachines should be able to provide such recovery discs and minimal cost),,,, OR...many eMachines computers also have a Recovery Partition which allow you to do a "non-destructive" recovery of the system (normally it doesn't damage any of your personal files) or a "destructive" recovery which formats the drive and reinstalls everything back to its factory condition. A "destructive" recovery will wipe all data from the hard drive including your personal information.
Depending on the make an model of your eMachines, the Recovery partition steps may be a little different.. Usually, the best method is to restart the computer and press the appropriate F-key to access the Recovery partition.. Or, sometimes, there is a "System Recovery" icon in the Start Menu.
Hope this helps.