A most likely cause of this stalling: the engine idle needs to be
raised when the engine is under load or cold...if not, a load like the
AC compressor can stall it. This is accomplished by the idle air
bypass valve. It is a solenoid valve (electrical) that is controlled
by the computer. It adds air to the engine intake - same as if you
opened the throttle plate. It is possible that yours is defective,
needs cleaning or is not getting the proper signal. Unforunately, at
least in the Zetec, it is in a very inconvenient location...lower part
of intake manifold. You may have to get it from underneath. I have not
had to remove mine yet.
This problem can happen to different cars and is no reason to be
afraid of yours. It is not a common Focus problem, from what I hear.
Get it fixed.