Why does the CPU cooling fan on my FIC laptop frequently switch off
when Windows XP Pro loads?

CPU fans are controlled by the motherboard, which reads temperature
readings from the CPU.  On a laptop, they are designed to shut off
whenever possible to minimize battery use.  During boot up,  your
system is loading data from the hard drive into memory and probing the
system for all available devices.  None of this is very CPU intensive,
so the motherboard decides to shut off the fan.  When it boots up, if
you start a CPU intensive task (playing a 3D video game, for example),
you'll quickly hear the fan kick back in.

Hope this helps!

does it just slow down, or completly stop when booting up?

Probably because loading the OS is a hard drive intensive operation,
not a CPU-intensive one.  Thus, the CPU isn't using as much energy,
and doesn't need the cooling.  Further, as XP loads, it will get to
the ACPI drivers, which allow it to throttle back the CPU speed and
voltage, which reduces power consumption as well.