Ok. I got it now. We only use one version of AutoCAD with no verticals, so that never really occurred to me. However, you could address that by doing the following:
1) Open an explorer window.
2) From the pulldowns, select Tools->Folder Options
3) Select the File Types tab
4) Scroll down till you find the DWG file type, and select it
5) Click the Advanced button
6) Add new actions to open DWG files with the desired programs
Then, when you right-click a DWG file in explorer, you will have options in your context menu to open it in whichever program you desire.