It is likely that the only program capable of STU to AVI conversion would be the one from which the *.stu files originate - Pinnacle Studio. As *.stu files are projects, it is unlikely any other program would be capable of somehow exporting or converting these projects.
Convert Pinnacle Studio video project to Audio Video Interleave movie.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert stu to avi on Windows
Pinnacle Studio is popular video editing application for Microsoft Windows. It is distributed as a standalone application as well as bundled software with some TV tuners for computers.
Convert from stu file
to avi file
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.