In order to convert to XviD, the easiest tool to use by far is "AutoGK" available at http://www.autogk.me.uk/
Since you have a *.ifo file, all you have to do is select "DVD Input" for 1st step. In this step you select the FOLDER containing the dvd files.

You select the output file name and location

For Step3 you choose the size of the output you wany, 2CD, 3CD etc

More options may be selected by clicking on the "Advanced Setting" tab.

Then Click on "Add Job"
Click on "start"
.. then wait for around 6-10 hours for task to complete :-)

-Shahed