If all you have done is added a CD(RW) to the system you need to check the following:
Have you set the pins on the CD(RW) to SLAVE mode if you are chaning it off the primary CD(RW) or hard drive or left it as PRIMARY if you are connecting it to the secondary IDE channel.
What IRQ number is your modem using - there may now be a conflict if the secondary IDE channel is now in use and it wasn't before.