We have a Dell OptiPlex 745 desktop system which will often fail to boot after a restart.  The diagnostic lights show a 1-2-3 pattern which has no definitive solution in the manual.  Pressing the power button shuts down the system immediately after this particular boot failure.  Pressing the power button from an power off condition always results in a successful boot.  This problem only occurs after a Windows restart and then not with every restart but intermittently.