I'd suggest taking Le Bus Direct, line 4, from the CDG terminal to Gare Montparnasse. There's one every half hour, and the trip takes about 75 minutes. It stops at Gare de Lyon on the way. The one-way fare is 17 euros, bought in advance on the website. You can print the ticket. It appears to be good for a year, you don't need a reservation so you don't have to worry if the flight is late or passport control takes awhile.
Most likely you'll be at Gare Montparnasse well before noon. Check the departure board for the trains to Chartres, which are frequent and take about an hour. Buy your ticket at the window, you don't need a reservation and you won't save with advance purchase. I don't know whether the station has a checked-bag facility, but if it does you could leave your carry-on there.
Then you have time for a nearby café lunch and maybe a stroll to the nearby Rue Daguerre market, or a visit to nearby Montparnasse Cemetery, or a quick trip up the nearby Montparnasse Tower to see the city (including the taller Eiffel Tower) from above. Get back to the station and the right track in time to make your train, and voila!
It would cost a little less, and could be a little faster, to transfer between CDG and Gare Montparnasse by RER "B" and Metro (see "Paris by Train" from the station link above). But I'd go for the convenience of the bus. Even a carry-on bag would make the rail option harder, and there's a pickpocket risk as well.
