Vista - Winndows installer

After installing Windows home premium, I still have a few issues over that I
then installed Firefox, as I like to use this browser sometimes for specific
reasons. It appeared to install okay and then something went wrong, I can't
remember what it was so that I proceeded to uninstall it. Ever since then
every time I try and install something I get the message telling me that the
previous installation had not completed and that the installation should be
finished before anything else can be installed.

I have looked to see whether Firefox is still there but I can't find it
anywhere so there is nothing for me to either uninstall all complete and
installation. I have absolutely no idea what to do, does anybody have any
suggestions?