You may have different versions of AutoCAD (2004, 2007, 2008) installed on your system as well as AutoCAD LT 2006 and AutoCAD Architecture 2009. All of these applications open .DWG files. If you want a specific program to open a file that you have selected in Windows Explorer simply edit the following regisry key:

"C:\Program Files\AutoCAD 2010\acad.exe"

The \AutoCAD 2010\acad.exe directory defines which application will be used as the default application.