OK, I am confused. You talk about AA X Pro and also AA XI Pro. Which are you trying to install. Generally you would not have both AA X Std and Pro on the same machine -- Pro has everything of Std and there is no need and likely conflict. You need to check to see what is installed (on Windows go to the Control Panel>Programs & Features and see what is listed).
A NOTE: If you ever have to reinstall AA XI in the future, you will need to S/N of X that is used for the upgrade. You also need to be sure you have saved the XI install file (and all the S/Ns -- X and XI) to a backup HD and/or CD for future use.
With this in mind, I assume you want to install AA XI. So, just deactivate and uninstall BOTH versions of AA X. (Again, you will need the S/Ns.) Then run http://labs.adobe.com/downloads/acrobatcleaner.html. The reboot and install. After the install, do the updates (Help>Updates -- only the latest needed for XI). In the process you will have to provide the S/N of the qualifying product and the new S/N. I strongly recommend that you register your product at Adobe as you go.