AutoCAD 2008 with Windows 7 > Every Drawing Opens a New Session

Hi All,
I am using Inventor 2008 and recently switched over to Windows 7 from
XP. The problem I'm having is this, when I double click a dwg from
explorer, Inventor launches instead of AutoCAD. No problem, I'll just
default dwg's to open with AutoCAD instead of the DWG Launcher
application. That works fine; but now when I open from explorer, I get
a new session of AutoCAD for every drawing I open. I want each drawing
to open in the same session, this prevents a million tabs in the
taskbar and all my macros from having to reload every time. I need to
open a lot of drawings quickly throughout the day, so the file>open
routine is not a great option for me; although it does solve this
problem. Does anyone know how to fix this problem? Thanks, Paul.