plz help me with emachines! i need product key?

I have an Emachines computer that needs a product key, which i don't have. this does not let me use the computer unless I have the product key, or re-instal the computer. The model is : emachines T2778, microsoft windows xp, desktop. i don't know what to do, it's been a while since this computer has been asking me to acivate windows (that's where i have to put the product key). i have changed the hard disk a couple of times, maybe that's where the problem is....plz help :(