There can be many different causes to DCS startup failures and crashes-to–desktop: a genuine DCS bug, conflicting user mods, incorrect DCS settings, faulty drivers, hardware failures, a corrupted file system...
This guide is here to help. If it fails to do so, create your own thread and attach your dcs.log file.
A. Windows instantly shows an error popup ("Bad image" or "The application was unable..." or "Error 0xC0000142")
B. DCS crashes before the main menu is reached