Most install/uninstall problems are due to remnants of older versions and install/uninstall debris blocking normal install processes. Until you completely clean your system of any older versions, broken install or incomplete uninstall debris, you won't be able to install a new version or
program. Vista uninstaller does a very poor job of removing all parts of a program.
You can do a manual uninstall with the help of Windows Installer CleanUp Utility, but the best way to get rid of all remnants of an older program, failed download and install, or failed uninstall is using Revo Uninstaller or Smarty uninstaller.
REVO UNINSTALLER
SMARTY UNINSTALLER
You can access a number of proprietary removal tools and instructions for doing a manual uninstall at the tutorial link below.
Install/Uninstall Problems With Vista