Uninstall previous install when running Windows installer

installer/picard-setup.nsi.in

@@ -184,16 +184,34 @@ Function un.onInit
 FunctionEnd
 
 Function .onInit
+  ; Abort installation if this is not a 64 bit system
   ${IfNot} ${RunningX64}
     MessageBox MB_ICONSTOP|MB_OK "This version of ${PRODUCT_NAME} requires a 64-bit Windows system."
     Abort
   ${EndIf}
 
+  ; Abort installation if Picard is currently running
   FindWindow $0 "${WNDCLASS}" "${WNDTITLE}"
   StrCmp $0 0 continueInstall
     MessageBox MB_ICONSTOP|MB_OK "The application you are trying to install is running. Close it and try again."
     Abort
   continueInstall:
+
+  ; Uninstall previous installation
+  ReadRegStr $R0 HKLM "${PRODUCT_UNINST_KEY}" "UninstallString"
+  StrCmp $R0 "" skipUninstall
+
+  MessageBox MB_OKCANCEL|MB_ICONEXCLAMATION \
+  "${PRODUCT_NAME} is already installed. $\n$\nClick `OK` to remove the \
+  previous version or `Cancel` to cancel this upgrade." \
+  IDOK uninstall
+  Abort
+
+  uninstall:
+  ClearErrors
+  Exec $R0
+
+  skipUninstall:
   !insertmacro UnselectSection ${desktop}
 FunctionEnd