One of my machines is an older Dimension 8400.  I added an aftermarket DVD drive as divve E: a couple of years ago.  The windows registry is corrupt and I need to repair Vista.  To do so I need to boot from the Vista disk which is a DVD.  The only boot order I can see in system setup is C: followed by D: which is a CD only drive.  How can I get it to boot from the DVD drive?  Do I have to switch some cables and/or jumpers inside the box?  I am reasonably capable but by no means an expert on this stuff.  At a minimum would just like to repair it enough to salvage some data.  Thoughts?