I have a Compaq Presario CQ42 laptop. Can I add a 3G modem?
your system is shown linux so it would be kind if you tell which destro you are using and which brand of modem you want to use..it would be easy to answer
or simply connect the modem and check the notification area in the connection icon click and see weather its detected or not
Yes,you can use 3g modem in your laptop by simply inserting 3G USB dongle into the USB port of the laptop.
Beside a external card, like a usb or pcmcia/express card, there is no easy or simple way to install a card inside the computer.