diff --git a/windows/pageant.c b/windows/pageant.c
index 9c7a5696..221154e2 100644
--- a/windows/pageant.c
+++ b/windows/pageant.c
@@ -531,7 +531,7 @@ static void win_add_keyfile(Filename *filename, bool encrypted)
     }
 
   error:
-    message_box(traywindow, err, APPNAME, MB_OK | MB_ICONERROR,
+    message_box(traywindow, err, APPNAME, MB_OK | MB_ICONERROR, false,
                 HELPCTXID(errors_cantloadkey));
   done:
     sfree(err);
diff --git a/windows/platform.h b/windows/platform.h
index 7a333a0e..d14ea50e 100644
--- a/windows/platform.h
+++ b/windows/platform.h
@@ -398,8 +398,8 @@ bool request_file(filereq *state, OPENFILENAME *of, bool preserve, bool save);
 filereq *filereq_new(void);
 void filereq_free(filereq *state);
 void pgp_fingerprints_msgbox(HWND owner);
-int message_box(HWND owner, LPCTSTR text, LPCTSTR caption,
-                DWORD style, DWORD helpctxid);
+int message_box(HWND owner, LPCTSTR text, LPCTSTR caption, DWORD style,
+                bool utf8, DWORD helpctxid);
 void MakeDlgItemBorderless(HWND parent, int id);
 char *GetDlgItemText_alloc(HWND hwnd, int id);
 void split_into_argv(char *, bool includes_program_name,
diff --git a/windows/putty.c b/windows/putty.c
index f3cf31d0..9c8ba001 100644
--- a/windows/putty.c
+++ b/windows/putty.c
@@ -83,7 +83,7 @@ void gui_term_process_cmdline(Conf *conf, char *cmdline)
                 s2 = dupprintf("%s Warning", appname);
                 if (message_box(NULL, s1, s2,
                                 MB_YESNO | MB_ICONWARNING | MB_DEFBUTTON2,
-                                HELPCTXID(option_cleanup)) == IDYES) {
+                                false, HELPCTXID(option_cleanup)) == IDYES) {
                     cleanup_all();
                 }
                 sfree(s1);
diff --git a/windows/puttygen.c b/windows/puttygen.c
index 0c4b279b..bfd732ac 100644
--- a/windows/puttygen.c
+++ b/windows/puttygen.c
@@ -1139,7 +1139,7 @@ void load_key_file(HWND hwnd, struct MainDlgState *state,
         char *msg = dupprintf("Couldn't load private key (%s)",
                               key_type_to_str(type));
         message_box(hwnd, msg, "PuTTYgen Error", MB_OK | MB_ICONERROR,
-                    HELPCTXID(errors_cantloadkey));
+                    false, HELPCTXID(errors_cantloadkey));
         sfree(msg);
         return;
     }
@@ -1202,7 +1202,7 @@ void load_key_file(HWND hwnd, struct MainDlgState *state,
     if (ret == 0) {
         char *msg = dupprintf("Couldn't load private key (%s)", errmsg);
         message_box(hwnd, msg, "PuTTYgen Error", MB_OK | MB_ICONERROR,
-                    HELPCTXID(errors_cantloadkey));
+                    false, HELPCTXID(errors_cantloadkey));
         sfree(msg);
     } else if (ret == 1) {
         /*
@@ -1241,7 +1241,7 @@ void add_certificate(HWND hwnd, struct MainDlgState *state,
         char *msg = dupprintf("Couldn't load certificate (%s)",
                               key_type_to_str(type));
         message_box(hwnd, msg, "PuTTYgen Error", MB_OK | MB_ICONERROR,
-                    HELPCTXID(errors_cantloadkey));
+                    false, HELPCTXID(errors_cantloadkey));
         sfree(msg);
         return;
     }
@@ -1254,7 +1254,7 @@ void add_certificate(HWND hwnd, struct MainDlgState *state,
                        &error)) {
         char *msg = dupprintf("Couldn't load certificate (%s)", error);
         message_box(hwnd, msg, "PuTTYgen Error", MB_OK | MB_ICONERROR,
-                    HELPCTXID(errors_cantloadkey));
+                    false, HELPCTXID(errors_cantloadkey));
         sfree(msg);
         strbuf_free(pub);
         return;
@@ -1267,7 +1267,7 @@ void add_certificate(HWND hwnd, struct MainDlgState *state,
         char *msg = dupprintf("Couldn't load certificate (unsupported "
                               "algorithm name '%s')", algname);
         message_box(hwnd, msg, "PuTTYgen Error", MB_OK | MB_ICONERROR,
-                    HELPCTXID(errors_cantloadkey));
+                    false, HELPCTXID(errors_cantloadkey));
         sfree(msg);
         sfree(algname);
         strbuf_free(pub);
@@ -1295,7 +1295,7 @@ void add_certificate(HWND hwnd, struct MainDlgState *state,
     if (!match) {
         char *msg = dupprintf("Certificate is for a different public key");
         message_box(hwnd, msg, "PuTTYgen Error", MB_OK | MB_ICONERROR,
-                    HELPCTXID(errors_cantloadkey));
+                    false, HELPCTXID(errors_cantloadkey));
         sfree(msg);
         strbuf_free(pub);
         return;
@@ -1311,7 +1311,7 @@ void add_certificate(HWND hwnd, struct MainDlgState *state,
     if (!newkey) {
         char *msg = dupprintf("Couldn't combine certificate with key");
         message_box(hwnd, msg, "PuTTYgen Error", MB_OK | MB_ICONERROR,
-                    HELPCTXID(errors_cantloadkey));
+                    false, HELPCTXID(errors_cantloadkey));
         sfree(msg);
         return;
     }
diff --git a/windows/utils/message_box.c b/windows/utils/message_box.c
index ae78de4a..11b69794 100644
--- a/windows/utils/message_box.c
+++ b/windows/utils/message_box.c
@@ -1,5 +1,9 @@
 /*
- * Message box with optional context help.
+ * Enhanced version of the MessageBox API function. Permits enabling a
+ * Help button by setting helpctxid to a context id in the help file
+ * relevant to this dialog box. Also permits setting the 'utf8' flag
+ * to indicate that the char strings given as 'text' and 'caption' are
+ * encoded in UTF-8 rather than the system code page.
  */
 
 #include "putty.h"
@@ -25,10 +29,10 @@ static VOID CALLBACK message_box_help_callback(LPHELPINFO lpHelpInfo)
         launch_help(message_box_owner, context);
 }
 
-int message_box(HWND owner, LPCTSTR text, LPCTSTR caption,
-                DWORD style, DWORD helpctxid)
+int message_box(HWND owner, LPCTSTR text, LPCTSTR caption, DWORD style,
+                bool utf8, DWORD helpctxid)
 {
-    MSGBOXPARAMS mbox;
+    MSGBOXPARAMSW mbox;
 
     /*
      * We use MessageBoxIndirect() because it allows us to specify a
@@ -37,13 +41,31 @@ int message_box(HWND owner, LPCTSTR text, LPCTSTR caption,
     mbox.cbSize = sizeof(mbox);
     /* Assumes the globals `hinst' and `hwnd' have sensible values. */
     mbox.hInstance = hinst;
-    mbox.hwndOwner = message_box_owner = owner;
-    mbox.lpfnMsgBoxCallback = &message_box_help_callback;
     mbox.dwLanguageId = LANG_NEUTRAL;
-    mbox.lpszText = text;
-    mbox.lpszCaption = caption;
-    mbox.dwContextHelpId = helpctxid;
+
+    mbox.hwndOwner = message_box_owner = owner;
+
+    wchar_t *wtext, *wcaption;
+    if (utf8) {
+        wtext = decode_utf8_to_wide_string(text);
+        wcaption = decode_utf8_to_wide_string(caption);
+    } else {
+        wtext = dup_mb_to_wc(DEFAULT_CODEPAGE, 0, text);
+        wcaption = dup_mb_to_wc(DEFAULT_CODEPAGE, 0, caption);
+    }
+    mbox.lpszText = wtext;
+    mbox.lpszCaption = wcaption;
+
     mbox.dwStyle = style;
+
+    mbox.dwContextHelpId = helpctxid;
     if (helpctxid != 0 && has_help()) mbox.dwStyle |= MB_HELP;
-    return MessageBoxIndirect(&mbox);
+    mbox.lpfnMsgBoxCallback = &message_box_help_callback;
+
+    int toret = MessageBoxIndirectW(&mbox);
+
+    sfree(wtext);
+    sfree(wcaption);
+
+    return toret;
 }
diff --git a/windows/utils/pgp_fingerprints_msgbox.c b/windows/utils/pgp_fingerprints_msgbox.c
index 6618de82..b0733008 100644
--- a/windows/utils/pgp_fingerprints_msgbox.c
+++ b/windows/utils/pgp_fingerprints_msgbox.c
@@ -21,5 +21,5 @@ void pgp_fingerprints_msgbox(HWND owner)
         ", " PGP_PREV_MASTER_KEY_DETAILS "):\n"
         "  " PGP_PREV_MASTER_KEY_FP,
         "PGP fingerprints", MB_ICONINFORMATION | MB_OK,
-        HELPCTXID(pgp_fingerprints));
+        false, HELPCTXID(pgp_fingerprints));
 }