diff --git a/os/windows/installer/install.nsi b/os/windows/installer/install.nsi
--- a/os/windows/installer/install.nsi
+++ b/os/windows/installer/install.nsi
@@ -102,6 +102,9 @@ Page custom SelectCDEnter SelectCDExit "
 ;--------------------------------------------------------------
 ; (Core) OpenTTD install section. Copies all internal game data
 Section "!OpenTTD" Section1
+	; Make sure to be upgraded OpenTTD is not running
+	Call CheckOpenTTDRunning
+
 	; Overwrite files by default, but don't complain on failure
 	SetOverwrite try
 
@@ -517,7 +520,6 @@ Abort:
 Done:
 FunctionEnd
 
-
 ;-------------------------------------------------------------------------------
 ; Check whether we're not running an installer for NT on 9x and vice versa
 Function CheckWindowsVersion
@@ -537,6 +539,23 @@ Abort:
 Done:
 FunctionEnd
 
+;-------------------------------------------------------------------------------
+; Check whether OpenTTD is running
+Function CheckOpenTTDRunning
+	IfFileExists "$INSTDIR\openttd.exe" 0 Done
+Retry:
+	FindProcDLL::FindProc "openttd.exe"
+	Pop $R0
+	IntCmp $R0 1 0 Done
+	ClearErrors
+	Delete "$INSTDIR\openttd.exe"
+	IfErrors 0 Done
+	ClearErrors
+	MessageBox MB_RETRYCANCEL|MB_ICONEXCLAMATION "OpenTTD is running. Please close it and retry." IDRETRY Retry
+	Abort
+Done:
+FunctionEnd
+
 Var OLDVERSION
 Var UninstallString