Trying to use Add/remove programs yet when I select a program to install the "continue" box is greyed out.

When I open Add/remove programs and try to select a program that I want to install from the list (eg. Thunderbird) I get the description of the sofware on the window on the right yet the box that should allow me to continue at the bottom of this box is greyed out and does not allow me to proceed with the install.Does not work with any of the listed programs.