Receiving "Please insert CD error" when trying to install xubuntu desktop in 12.04

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http://askubuntu.com – I have installed Ubuntu 12.04. I want to install xfce desktop. When I tried to install Xubuntu desktop, I got the following message. sudo apt-get install xubuntu-desktop Media change: please insert the disc labeled 'Ubuntu 12.04.1 LTS _Precise Pangolin_ - Release i386 (20120817.3)' in the drive '/cdrom/' and press enter I didn't know why I'm getting this error, and also I don't have CD drive. I installed Ubuntu using usb. Please let me know how to install xubuntu desktop. (HowTos)