One overlooked reason for getting access denied and/or other problems when installing software is the existence of a file that needs to be replaced but can't be due to a read-only attribute. This most often occurs in the %SystemRoot% directory and its' sub-directories. To display the files in your NT directory that have the read-only attribute, type:

dir %systemroot%\*.* /ar /s

You can remove the read-only attribute from a file by using the attrib command. To remove the read-only attribute from all files in the NT directory and sub-directories, type:

attrib -r %systemroot%\*.* /s