In order to install Windows ME, you must know the product key of the Windows ME CD - ROM that you have bought. Windows ME is similar with Windows XP, in terms of activation. You must activate Windows ME in order for you to continue to use it.
Windows ME Does NOT require activation, and it is like XP in that it is like all the other Windows OS's to date with product keys: ANY product key for that particular edition will work with ANY XP CD (not counting of course recovery CDs). I was fairly sure that it didn't, but I double-checked with google. First hit confirmed that ME does NOT have an activation process.
Actually, I was 100% sure, otherwise XP would not have been the First to use it, now would it?