Are you using the portable version of screenshot captor or the installer version?
You say the error happens on computer startup. If you exit Screenshot Captor and restart it manually, do you not get the error? Or does it happen whenever you start screenshot captor?
The first thing you might try is to exit SC and delete the ScreenshotCaptor.ini file to reset any of your screenshot captor options and settings, and see if that fixes it.
If you didn't install SC normally (for example if you copied it over from another machine), you might try uninstalling or deleting it and reinstalling it.
Another thing to try is to use the portable version and see if that has the same problem -- if not then we know it has something to do with where/how it was installed.