You don't need to set up a file type to open your .bak files. Just double click on the .bak file and select RENAME, then delete the bak and replace it with dwg, meaning that the file name ends with '.dwg' . Then when you click on it to open it, it should open with Autocad. If for some reason it is still trying to open with an inappropriate program, choose OPEN WITH and specify Autocad 2007.
When you try to reformat the file type, you will be advised that it may make the file unopenable, ignore that and continue changing the .bak file type to .dwg and then you will be able to open it.