Software flashes just a split second in taskbar. How to find out what software it is?

Process Monitor can achieve this:

Set the filter to "Operation is 'Process Create'" and "Operation is 'Process Start'" and watch events coming in:

A more complicated way to do this is to use the Group Policy editor and then activate "process auditing", but that's a bit more complicated than just using Process Monitor.

The problem was caused by a software by Huawei Mobile Partner (DiGi Net Mobile - my version), controller for my internet stick from my ISP. First of all RunLiveUpd.exe is the cause, I think triggering Interactive Services Detecion - UI0Detect.exe showing the icon in taskbar for a short amount of time.

Here is the screen shot of Process Monitor showing the processes that started: Finally I've chosen to reinstall DiGi Net Mobile, and the problem disappeared. Certainly this is some sort of a bug.