The bad news is that there are LOTS of possibilities to explore; the good news is that it is most likely shutting down a program that is running in the background. You can easily check this out by running one of the utilities that will display all the programs that are activated in the start up.
I like CCleaner by Piriform because it has a tool where you can easily disable start up files one by one to find the problem, and then easily reactivate the ones you want back.
You can also use the Windows system utility "msconfig" at the Run command, but I think it's a little messier to troubleshoot with.
If it's not a startup file, then the problem may be a Windows service
, which is beyond my ability to pin down.
But there are plenty of experts on this forum that can probably help you out.