|Which emachines model is it ?|
The Windows Product Key is on the official Microsoft label that should be on the outside of the computer case.
If that's there but the Product Key is worn so you can't read it, or if it's not there,....
To find an existing Product Key your Windows installation is using that you don't know...
If Windows is working on the emachines computer, search the web for: keyfinder
download it, run it.
If Windows is installed but not wokring properly (will not load), if you install the emachines drive on another computer, or connect it to another computer, such that you're not attempting to boot the computer from it, if the operating system on the other computer is in the same family as Windows on your hard drive - XP, Vista, or Windows 7 - you can use the Load Hive feature in the top menu of Keyfinder to find the Product Key of your Windows installation on your drive.
(More info about Load Hive in the FAQs on the official Keyfinder web site.)