update from 8.1 to XI failed 5 times; error message that 8.1 is running(not); windows xp, ver 2002

While trying to update from reader 8.1 to readerXI, install has failed 5 times after getting an error message that 8.1 is using files that need to be updated (it is not running): operating system is windows xp, version 2002.  Do I need to uninstall 8.1 before trying to update, and how can I be sure that XI will install correctly and not leave me holding an empty bag if I do this and the download still does not install