The reason I wonder is because *I THINK* all error messages that occur within AutoCAD have 'AutoCAD' in the titlebar. The msgbox you posted *LOOKS* like a Windows box...
Sorry, I'm not sure what else it could be...
Is the code the same in the Folder Options?
(Select DWG from the list of registered file types and click on the Advanced button. Under Actions, select Open and click on the Edit button. Then compare two machines against eachother. Maybe this is what you just did...just wanted to clarify...)
PS - I changed the title of your post to be a little more descriptive. Hope you don't mind.