.dwg files mistakenly opening in Illustrator

Hoping someone can help...

I recently installed AutoCad LT 2013 on a new iMac.  (Previously, I had been using AutoCad 2011 on an older iMac without any problems.)  A few problems:

 -  When I try to open a .dwg file, the file is being read as an "autocad image file" and is opening in Adobe Illustrator.  The only way to open a file is from within AutoCad.  For now, I've uninstalled Illustrator, but since I use this software, this is not a longterm solution.

-  Many of my files will only open as "read only" files even though they are not being accessed by any other computer, and are not open on my computer.

Any insight would be much appreciated.  

Thanks!