1.) First step is to reinstall RadarBox 2009, this is incase of any corruption in the software.
Can I save my DB files, and if so which files do I need to save to keep the software happy?
2.) Secondly are the aircraft which never get removed ADS-B aircraft, or are they always Mode-S aircraft?
I think usually Mode-S, I will double check.
If so is it always Northwest?
No, I have seen Continental and Sun Country so far.
3.) Do you have another machine where you can run RB on, this is so we can see whether its just related to a specfic setup or whether it can be replicated which will help us track down the issue.
I can change quickly to a different System for testing.