PC-cillin r-t scanning and WinXP

I have many copies of PC-cillin 2002 that came bundled
with Asus motherboards and I've installed it on many
computers. But sometimes when I install it with Win XP,
it displays an error message on bootup : "Real-time
Scanning service not started. Do you want to start it?".

It's only with Win XP and only with some installations.
Uninstalling and re-installing PC-cillin makes no difference.
The real-time monitor option is always enabled by default
even with those installations that display the error message.

I can't think of anything I did differently between those
installations that display the error message and those that
don't, except perhaps the order in which I installed all the
different softwares I normally install. And I'd hate having
to install Win XP and all those apps and utilities over
and over again just to find out.

Can anyone please throw some light on the matter ?