I think if you download the update that Arjan gave (WaveLab 6.1.1) it will probably install a suitable update for the eLicenser version on your XP machine as well. If you then purchase the upgrade to WL8.5 (which will not install on XP!), you can install WL8.5 on your Windows 8.1 laptop, and from there you can enter the activation code that will be emailed to you on purchase (as described in the video on the eLicenser website) and all should be well.
One word of caution, when you get your activation code do not post it here on or any public forum; think of it as a one-time password which can only be used by you to activate your personal licence which is then physically stored on your USB eLicenser.