Depending on the size of your installation, you can use either Remastersys http://geekconnection.org/remastersys/ubuntu.html
, or Clonezilla http://clonezilla.org/
Remastersys is a more intuitive program to use, but it has limitations. If you are using proprietary drivers for your graphics card or wireless, they must be uninstalled before Remastersys is run and reinstalled in the new installation. Any symlinks you have must be removed or Remastersys will try to follow them and copy all data in the remote files. Finally, Remastersys has great difficulty handling more than 4 GB of files in one .iso.
That said, Remastersys can do quite a good job as long as the limits are not exceeded. Just be sure, if you decide to try it to download the Debian version, not the Ubuntu version.
I am not as familiar with Clonezilla. It is a larger program than Remastersys and can do much more. I would suggest reading the information on their website (see link above) to find out more about Clonezilla and its capabilities.