diff --git a/Code/Mantid/Build/CMake/NSIS.template.in b/Code/Mantid/Build/CMake/NSIS.template.in
index 25b5bd197485624e394dd0601f3cb573d3e257cd..82c7714fbf9b57854bac25d196b4baf16dd6c45f 100644
--- a/Code/Mantid/Build/CMake/NSIS.template.in
+++ b/Code/Mantid/Build/CMake/NSIS.template.in
@@ -36,6 +36,8 @@
 
   ;Set compression
   SetCompressor @CPACK_NSIS_COMPRESSOR@
+  
+  SetOverwrite @CPACK_NSIS_FORCE_OVERWRITE@
 
 @CPACK_NSIS_DEFINES@   
 
diff --git a/Code/Mantid/Build/CMake/WindowsNSIS.cmake b/Code/Mantid/Build/CMake/WindowsNSIS.cmake
index be20e9e306879ff15be2868db55af90cea868ca5..dcaff89c49072df6a85ead64d8b6156a441ed3de 100644
--- a/Code/Mantid/Build/CMake/WindowsNSIS.cmake
+++ b/Code/Mantid/Build/CMake/WindowsNSIS.cmake
@@ -14,8 +14,10 @@
 	set( CPACK_INSTALL_PREFIX "/")
 	set( CPACK_NSIS_DISPLAY_NAME "Mantid${CPACK_PACKAGE_SUFFIX}")
 	set( CPACK_PACKAGE_NAME "Mantid${CPACK_PACKAGE_SUFFIX}" )
-	set( CPACK_PACKAGE_INSTALL_DIRECTORY "MantidInstall${CPACK_PACKAGE_SUFFIX}")
+	set( CPACK_PACKAGE_INSTALL_DIRECTORY "MantidInstall${CPACK_PACKAGE_SUFFIX}") 
 	set( CPACK_NSIS_INSTALL_ROOT "C:")
+	set( CPACK_NSIS_FORCE_OVERWRITE on CACHE STRING "Force installer to overwrite any existing install")
+	set_property(CACHE CPACK_NSIS_FORCE_OVERWRITE PROPERTY STRINGS on off)
 
 	#set( CPACK_NSIS_ENABLE_UNINSTALL_BEFORE_INSTALL "ON")
 	set( CPACK_PACKAGE_ICON "${CMAKE_CURRENT_SOURCE_DIR}/Images\\\\MantidPlot_Icon_32offset.png" )