The easiest way to find the source, assuming the error dialog is produced by a program running on startup, may be to use my Home Folder program. Go to my page
Miles Ahead's Windows Downloads
search on HomeFolder and download.
Extract to an empty folder and see Readme.txt for details.
But the short of it is, run HomeFolder and it sits in the tray.
When this error dialog pops up make sure it is the active window
(for instance click the mouse on the caption bar of the dialog)
and press Shift Numpad Minus hotkey combination. If HomeFolder can
find the exe that owns the window it will open the folder where the
exe file is. This should give a good clue where the problem lies.
As example if notepad.exe is the active window and I press the HomeFolder hotkey, it should open the folder C:\Windows since that is where notepad.exe file is.
Edit: since you are getting the error on startup, just let the error dialog sit there until you can run HomeFolder. Once you see it is in the System Tray, then you can hit the hotkey to open the home folder
of the program that launches the error dialog. If it is a system error that might be C:\Windows\System32. But if it is a program error dialog then it should narrow it down to the exe with the problem.