X-post, Fup2: misc.kids.computer

I am looking for a utility for Windows XP which
can control which programs can at one time be executed.

While I want my son to use his educational software etc,
I want to be able to turn installed games etc off at that time
and only grant access for certain periods of time (not even automated).

On a UNIX system I'd just write a shell-script to set the permissions
or such, but on Windows?

Ideally, the utility should also be usable (ie turn on-off) by Mom,
so an easy interface should be included. (I can configure the
settings, they must simply be switchable by Mom..)

There seems to be an option to have a separate account with
access to the games and one for school work etc, but that does
not sound like a good solution. I hope there is something better:

a) group games into categories
b) turn on/off categories
c) optionally a time-limiting function

Any hints, pointers etc appreciated.

Also, hints for other tools parents have found useful (other than
actual games, educational software which is OT for *this* thread) for
managing their children's computer usage would be appreciated..

I can summarize to the group (misc.kids.computer) what I learn

regards
Mathias