diff --git a/FreenetInstall_InnoSetup.iss b/FreenetInstall_InnoSetup.iss
index 9de4109..839bc0f 100644
--- a/FreenetInstall_InnoSetup.iss
+++ b/FreenetInstall_InnoSetup.iss
@@ -300,3 +300,22 @@ begin
   if (PageID = JavaDependency.Page.ID) And fCheckJavaInstall() then Result := True;
   if (PageID = NetDependency.Page.ID) And IsNetInstalled() then Result := True;
 end;
+
+function IsFreenetInstalled(): Boolean;
+var
+  RegPath: String;
+begin
+  Result := False;
+  RegPath := ExpandConstant('Software\Microsoft\Windows\CurrentVersion\Uninstall\{#emit SetupSetting("AppId")}_is1');
+  if RegValueExists(HKLM, RegPath, 'UninstallString') or RegValueExists(HKCU, RegPath, 'UninstallString')
+  then Result := True;
+end;
+
+function InitializeSetup(): Boolean;
+begin
+  Result := True;
+  if (IsFreenetInstalled()) then begin
+    Result := False;
+    MsgBox(CustomMessage('FreenetAlreadyInstalled'), mbCriticalError, MB_OK);
+  end;
+end;
diff --git a/translations/Messages_en.isl b/translations/Messages_en.isl
index c4b04c4..7d8833f 100644
--- a/translations/Messages_en.isl
+++ b/translations/Messages_en.isl
@@ -8,6 +8,7 @@ DependencyInstalled=%1 has been installed on your system.
 ErrorLaunchDependencyInstaller=Can't launch %1 installer.%n%nError (%2): %3.
 AdditionalOptions=Additional options:
 StartFreenetWithWindows=Start Freenet when I log in to Windows
+FreenetAlreadyInstalled=An existing Freenet installation is found. This installer cannot be used to update Freenet to a newer version. To update, enable Freenet's built-in automatic updates or run 'update.cmd' from the installation directory. To re-install, please uninstall Freenet first.
 
 [Messages]
 WelcomeLabel2=This will install [name/ver] on your computer.
diff --git a/translations/Messages_en_utf8.isl b/translations/Messages_en_utf8.isl
index c49b859..1299cc7 100644
--- a/translations/Messages_en_utf8.isl
+++ b/translations/Messages_en_utf8.isl
@@ -8,6 +8,7 @@ DependencyInstalled=%1 has been installed on your system.
 ErrorLaunchDependencyInstaller=Can't launch %1 installer.%n%nError (%2): %3.
 AdditionalOptions=Additional options:
 StartFreenetWithWindows=Start Freenet when I log in to Windows
+FreenetAlreadyInstalled=An existing Freenet installation is found. This installer cannot be used to update Freenet to a newer version. To update, enable Freenet's built-in automatic updates or run 'update.cmd' from the installation directory. To re-install, please uninstall Freenet first.
 
 [Messages]
 WelcomeLabel2=This will install [name/ver] on your computer.