The following solution didn't work for me but it worked for the person who used it so here it is for anyone who's as unlucky as me to find their DJ not working:
Dell DJ20 Recovery Mode
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I posted this question and then played around and fixed it using the "fixup.zip" file located in this part of the website:
http://www.delldjsite.com/forums/showpost.php?p=60383&postcount=6
I extracted it and followed its instructions and the computer recognized my Dell DJ. I was then able to re-install old firmware and it is ready to go now!
Many thanks that I was helped by that!
Andrew
January 2008