If you want a high quality "master" to go back to, first render as and AVI
File | Render As...
Save as type...Video for Windows (*.avi)
Template ...PAL DV (or DV Widescreen) ...(or NTSC alternatives if North America)

And that's it. If you now delete your project but save the avi created above you can simply create a new project and drop the avi on the timeline.

To render for DVDA...
File | Render As....
Save as type....MainConcept MPEG-2
Template....DVD Architect PAL video stream (or widescreen, or NTSC equivalent...)
and the defaults under the custom button will do fine.

THEN YOU ALSO NEED TO ENCODE THE AUDIO...
File | Render As..
Save As type Dolby Digital AC-3 Pro (*.ac3)
and leave the template as default.

(It may seem tedious to render the sound separately, but it generally only takes a few seconds)

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Tim