programs and updates install to external hard drive instead of internal sata

I am having a problem with unwanted installations to external hard drive when it seems i have no control over where program or microsoft updates install. actually the worst problem was when windows live updates and installations went to external. i did not discover this until recently and this explained why i have had to reinstall vista several times. i can always unplug the external if necessary but i would rather not. i asked western digital why this happens and they told me that since my internal hard drive is smaller than external that vista seems to not have solved this installation glitch yet. i prefer to use my external as storage for files and backups etc. does anyone know how to make my system recognize main drive as the smaller one. i hope this is clear enough to merit an answer. i have already suffered through a 3 day phone help with tech finally telling me to reinstall vista, which i did and it solved several other problems but not the int/ext installation issues.
thank you
fran

I would recommend backing up your data to the external drive. Shutting down. Unplugging the external drive. Reinstalling Vista on a clean setup. Boot up with the DVD and remove the partitions from your internal drive. Shutting down. Turn on and install starting fresh.

Thank you
i hope i am using this forum correctly. i am new. anyway i had followed that procedure the last time i reinstalled, so maybe i will wait and see what happens in the future. Western Digital tech had told me it would continue to be a problem but maybe he was wrong. or not aware of vista improvements. thanks again for the very speedy reply.
fran