Try restarting the computer, and then make sure that all programs are closed/disabled when you run the installer, including any anti-virus software.

If you are installing EndNote 7 or earlier and the installer is stopping at 89% or later, this last portion of setup is attempting to install the tools into Word. On some computers, the permissions are set so that programs are unable to access the folders EndNote needs to properly install the tools to Word. In these situations, you can always do a custom installation, not include the CWYW/Word addin files, and add them manually afterwards by following this FAQ.