Quote - "Must not have previously won a Renderosity Contest"   That's worded very generally.  Since I won the 2013 T-shirt design contest, does that mean I'm ineligable? 

I took a quick look, at one of the pages and that line appears under the entry specifications portion of the rules, not the general entry rules.

If you look at the section on entry specifications rules, they relate to the piece of work that you are submitting, not the user. As long as you have not entered or previously won a competition with that piece of work you should be ok.

User restrictions such as administrators etc are mentioned in the general entry section.

Daz Studio 4.8 and 4.9beta, Blender 2.78, Sketchup, Poser Pro 2014 Game Dev SR5 on Windows 8 Pro x64. Poser Display Units are inches