This may be an Ownership/Permissions issue or an install/uninstall issue.
Install/Uninstall Problems
Until you clean your system of any broken install or incomplete uninstall
debris, you won't be able to install a new program. Vista uninstaller does a
very poor job of removing all parts of a program. Sometimes a proprietary
removal tool can help, but even these will leave blocking files and registry
entries on your system.
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 a program, failed
download and install, or failed uninstall is using Smarty Uninstaller. You
can access Smarty Uninstaller, a number of proprietary removal tools and
instructions for doing a manual uninstall at the tutorial link below.
http://www.vistax64.com/tutorials/87249-unable-install-latest-version-program-vista.html
Take Ownership/Permissions
Even running as Administrator may leave you without permissions to modify
some files or folders, even ones that you have downloaded and installed. To
access or modify (copy, delete, rename) files or folders you can take
Ownership of (in your Username) and grant yourself Full Control permissions
of the files or folders in question described in the following tutorial.
To avoid having to do this individually for hundreds of files, Take
Ownership of large branches like Program Files folder, Users folder, or
Windows folder, and grant yourself Full Control of the folder, all subfolders
and files contained in the folder. In the case of Program Files, you would
have full access to all your programs. In the case of Users folder, you
would have full access to your Music, Pictures, Videos, Documents, etc. In
the case of Windows folder you would have full access to your OS, including
Windows Installer.
Windows Explorer Run As Administrator
Another way to gain access to modify files and folders includes going to the
Start button, typing in Windows Explorer, right-clicking on this and
selecting Run As Administrator. You should then have "elevated privileges"
to do whatever you wish to files in Windows Explorer.
Unlocker Program
There is also a program called Unlocker that you can use to gain access to
modify files and folders.
http://www.vistax64.com/tutorials/67717-take-ownership-file.html
http://ccollomb.free.fr/unlocker/