diff --git a/windows/CMakeLists.txt b/windows/CMakeLists.txt
index 6c802183..7bfa955c 100644
--- a/windows/CMakeLists.txt
+++ b/windows/CMakeLists.txt
@@ -5,6 +5,7 @@ add_sources_from_current_dir(utils
   utils/agent_named_pipe_name.c
   utils/arm_arch_queries.c
   utils/aux_match_opt.c
+  utils/centre_window.c
   utils/cryptoapi.c
   utils/defaults.c
   utils/dll_hijacking_protection.c
diff --git a/windows/dialog.c b/windows/dialog.c
index e68ef2af..65534815 100644
--- a/windows/dialog.c
+++ b/windows/dialog.c
@@ -348,7 +348,7 @@ static INT_PTR GenericMainDlgProc(HWND hwnd, UINT msg, WPARAM wParam,
                                   LPARAM lParam, void *ctx)
 {
     const int DEMO_SCREENSHOT_TIMER_ID = 1230;
-    HWND hw, treeview;
+    HWND treeview;
     struct treeview_faff tvfaff;
     int ret;
 
@@ -369,19 +369,8 @@ static INT_PTR GenericMainDlgProc(HWND hwnd, UINT msg, WPARAM wParam,
         }
         SendMessage(hwnd, WM_SETICON, (WPARAM) ICON_BIG,
                     (LPARAM) LoadIcon(hinst, MAKEINTRESOURCE(IDI_CFGICON)));
-        /*
-         * Centre the window.
-         */
-        {                              /* centre the window */
-            RECT rs, rd;
 
-            hw = GetDesktopWindow();
-            if (GetWindowRect(hw, &rs) && GetWindowRect(hwnd, &rd))
-                MoveWindow(hwnd,
-                           (rs.right + rs.left + rd.left - rd.right) / 2,
-                           (rs.bottom + rs.top + rd.top - rd.bottom) / 2,
-                           rd.right - rd.left, rd.bottom - rd.top, true);
-        }
+        centre_window(hwnd);
 
         /*
          * Create the tree view.
diff --git a/windows/platform.h b/windows/platform.h
index c6bf056a..25de278f 100644
--- a/windows/platform.h
+++ b/windows/platform.h
@@ -115,6 +115,8 @@ int ShinyDialogBox(HINSTANCE hinst, LPCTSTR tmpl, const char *winclass,
                    HWND hwndparent, ShinyDlgProc proc, void *ctx);
 void ShinyEndDialog(HWND hwnd, int ret);
 
+void centre_window(HWND hwnd);
+
 #ifndef __WINE__
 /* Up-to-date Windows headers warn that the unprefixed versions of
  * these names are deprecated. */
diff --git a/windows/utils/centre_window.c b/windows/utils/centre_window.c
new file mode 100644
index 00000000..651fc279
--- /dev/null
+++ b/windows/utils/centre_window.c
@@ -0,0 +1,20 @@
+/*
+ * Centre a window on the screen. Used to position the main config box.
+ */
+
+#include "putty.h"
+
+void centre_window(HWND win)
+{
+    RECT rd, rw;
+
+    if (!GetWindowRect(GetDesktopWindow(), &rd))
+        return;
+    if (!GetWindowRect(win, &rw))
+        return;
+
+    MoveWindow(win,
+               (rd.right + rd.left + rw.left - rw.right) / 2,
+               (rd.bottom + rd.top + rw.top - rw.bottom) / 2,
+               rw.right - rw.left, rw.bottom - rw.top, true);
+}