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AutoCAD drawings cant be open! Help!? - autocad lt 2005 safecastkg

I have about a month a project to complete on my laptop and now I can not open it! omg help me! I was leading with AutoCAD 2006 through to the end n then I have deleted the program (but not files), then I have installed AutoCAD LT 2005 to sing, and all open files. He says: "The file is invalid."
What's wrong with that? HELP!
I at 30 this month to present .. T_T helpppp