InstallAware

I suppose in my case it's because a dll is in use and can not be replaced.
But this dll has the same version and it is maked as 'shared' dll.

What else could set the REBOOT variable to true during installation?
I'd like to avoid rebooting.
Is there a way I can find out when it is set to true?

Of course, just read the REBOOT variable!

You can ignore its value, change your script, or do whatever you want in order to not reboot. But usually, you should allow reboots.

Reboots are determined internally by the Windows Installer engine, and can be caused by any number of things.

Ok I see, I formulate the question in another way:
I installed my software once. -ok
Now I uninstal the software... one dll remains because it is marked as shared dll and used by another programm. Thats fine so far..
If I now install my software again (the same version) the setup likes to reboot, why?
I suppose it could not replace the dll in use. But this is exactly the same dll.