My hard disk activity suddenly shoots up after I leave my
computer idle for 10 minutes or so. I don't know what the
hell it is doing. The Windows Task Manager shows
that 90% of processing resource is taken up by
"System Idle Activity"

It might be the antivirus (but it doesn't show like
that in Task manager)

I have Symantec Antivirus Corporate Edition installed. I
checked ever nook and corner for any "scheduled" activity
or similar kind of thing when the system is idle, but it looks
like it is not the antivirus.

I have ZoneAlarm, Spybot S&D, Adaware, etc but they don't
show anything suspicious.

The hard disk shows activity for a while and stops with I
move the mouse or press a key. It doesn't really interfere
with my work. I just want to know what the hell it is doing
and how I can stop it.

What could it be ? If it's the antivirus, where do I check it
in my Antivirus software because I couldn't find it..