Unable to install new programs to Vista

Hello,
My Vista Home Premium suddenly changed so that it does not install new programs unless the installation source is on the system drive C:
Depending on the program I try to install, I get error messages ranging from plain "installation cannot continue" to "cannot create temporary file".
This I have overcome by copying the installation source to C: but not all programs do allow it; must be installed from the original location.
Help, please.