DeVeDe is a really easy to use converter, and it´s a breeze to convert an Xvid-avi to a burnableDVD-iso for example. It´s in the Repos.
After you install it you will have to reinstall "mencoder" otherwise the sound will not work, but it is also very easy.
In Synaptic settings check the box for the Gutsy Backports repository, search for mencoder, rightclick it and choose upgrade ( i am at work at XPbox now, it might be reinstall also)
After it is installed, uncheck the backports box (unless you want the absolutely latest but perhaps buggy upgrades)