There really isn't an elegant fix for this problem. MS has been aware of this issue since the inception of DVD maker but they have refused to solve it:
Windows DVD Maker and Aspect Ratios - Dean Rowe's WebLog - Site Home - MSDN Blogs
"
You might think this will cause everything on the disc to be rendered as 4:3 or 16:9. However, as you may be aware, DVD’s allow a mixture of aspect ratios on a single disc. You can see this frequently on Hollywood discs where the main feature will be 16:9 and some of the extras may be 4:3.
We wanted DVD Maker to support generating DVD’s like these, so you can freely mix content of different aspect ratios on the same disc. When you add a video we will automatic detect its aspect ratio and render them appropriately either as 16:9 on the disc or as 4:3.
For DVD Menus, we will automatically generate them as 16:9 but we also specify the information so they can be rendered as 4:3 on a 4:3 display. This way the user doesn’t have to specify anything and they’ll get the desired behaviour of a menu that fills the screen"
So basically your out of luck because they wanted to give people the option of mixing aspect ratios on DVDs! That is the dumbest rationale but there you have it. Since 2006 MS has been bombarded with complaints from customers but they have staunchly ignored them all. If you don't believe me just do a google search for dvd maker aspect ratio problems or similar keywords.
There is a clumsy and time consuming work around using windows movie maker (NOT DVD maker):
This is to open windows movie maker, import the video file, make sure the 16:9 aspect ratio is set if you want that (project tab) then to save it> burn movie. This will change it from an avi file (what most people start off with) to an wmv file which is forcing the correct aspect ratio. This new file can be opened by DVD maker and burned properly.
The more elegant alternative is to use other software to go from avi to DVD such as convertXtoDVD. Until Microsoft decides to actually address this issue rather than ignoring it.
