diff --git a/mozilla/toolkit/xre/nsAppRunner.cpp b/mozilla/toolkit/xre/nsAppRunner.cpp
index 29b4764cd61..87b8812e95b 100644
--- a/mozilla/toolkit/xre/nsAppRunner.cpp
+++ b/mozilla/toolkit/xre/nsAppRunner.cpp
@@ -107,6 +107,11 @@
 
 #ifdef XP_WIN
 #include "nsIWinAppHelper.h"
+#include <windows.h>
+
+#ifndef PROCESS_DEP_ENABLE
+#define PROCESS_DEP_ENABLE 0x1
+#endif
 #endif
 
 #include "nsCRT.h"
@@ -2432,9 +2437,27 @@ static void MOZ_gdk_display_close(GdkDisplay *display)
  */
 PRBool nspr_use_zone_allocator = PR_FALSE;
 
+#ifdef XP_WIN
+typedef BOOL (WINAPI* SetProcessDEPPolicyFunc)(DWORD dwFlags);
+#endif
+
 int
 XRE_main(int argc, char* argv[], const nsXREAppData* aAppData)
 {
+#ifdef XP_WIN
+  /* On Windows XPSP3 and Windows Vista if DEP is configured off-by-default
+     we still want DEP protection: enable it explicitly and programmatically.
+     
+     This function is not available on WinXPSP2 so we dynamically load it.
+  */
+
+  HMODULE kernel32 = GetModuleHandleW(L"kernel32.dll");
+  SetProcessDEPPolicyFunc _SetProcessDEPPolicy =
+    (SetProcessDEPPolicyFunc) GetProcAddress(kernel32, "SetProcessDEPPolicy");
+  if (_SetProcessDEPPolicy)
+    _SetProcessDEPPolicy(PROCESS_DEP_ENABLE);
+#endif
+
   nsresult rv;
   ArgResult ar;
   NS_TIMELINE_MARK("enter main");