ntoskrnl.exe is a critical process in the boot-up cycle of the computer, that's why the warning pops up whenever you turn your computer on. The change can be due to a recent update. If you want to put your mind at ease, you can have it scanned
here or
here.
On top of the page there is a field to add the filepath, copy and paste this filepath:
C:\WINDOWS\system32\ntoskrnl.exe
Then hit Submit
The scan will take a while before the result comes up.