are you getting this error:
The Windows Installer Service could not be accessed.
This can occur if you are running Windows in safe
mode, or if the Windows Installer is not correctly
installed. Contact your support personnel for assistance.

well if so, this is how i was able to fix it, simply log out and log back in as administrator and try to re-install the program you were trying to install.

it happened to me, once i was able to run the installation with administrator, i didng have any more problems. i was trying to intall the java plugin for windows