I want to turn off factory installed alarm in 1999 Ford Taurus SHO but where is the switch located?

Hi there. As far as the valet/overide button, it's a hidden
device-switch, a little spring loaded micro button,
red or black possibly and is probably down under
the dash area, near your knee/leg. In some
remote start/alarm combos, the antenna on the window
glass will also have an override/valet button.
Older sytems will have a flip type toggle switch.

When you locate the button or switch,
turn on the ignition key, press and hold
in the button until the alarm chirps 1 time
(for toggle types, key on, flip switch in the
opposite direction, then key off). This should over ride the system. You would get a single chirp to confirm from the siren.
The LED (light emiting diode) light in the dash
should also go on solid. This is valet/overide/reset.
This should correct any issues like starter disable,
or anti-hijack. Your keyless entry, if installed,
will still be operable. The have it inspected for the
falsing issues you are experience at a local shop.
You would repeat the same steps to toggle back into user mode.