diff --git a/Modules/CMakeGenericSystem.cmake b/Modules/CMakeGenericSystem.cmake
index 7d2d8cdc8..b5d30724a 100644
--- a/Modules/CMakeGenericSystem.cmake
+++ b/Modules/CMakeGenericSystem.cmake
@@ -52,6 +52,94 @@ IF(CMAKE_GENERATOR MATCHES "Makefiles")
   ENDIF(DEFINED CMAKE_RULE_MESSAGES)
 ENDIF(CMAKE_GENERATOR MATCHES "Makefiles")
 
+
+# GetDefaultWindowsPrefixBase
+#
+# Compute the base directory for CMAKE_INSTALL_PREFIX based on:
+#  - is this 32-bit or 64-bit Windows
+#  - is this 32-bit or 64-bit CMake running
+#  - what architecture targets will be built
+#
+function(GetDefaultWindowsPrefixBase var)
+
+  # Try to guess what architecture targets will end up being built as,
+  # even if CMAKE_SIZEOF_VOID_P is not computed yet... We need to know
+  # the architecture of the targets being built to choose the right
+  # default value for CMAKE_INSTALL_PREFIX.
+  #
+  if("${CMAKE_GENERATOR}" MATCHES "Win64")
+    set(arch_hint "x64")
+  elseif("${CMAKE_SIZEOF_VOID_P}" STREQUAL "8")
+    set(arch_hint "x64")
+  elseif("$ENV{LIB}" MATCHES "(amd64|ia64)")
+    set(arch_hint "x64")
+  endif()
+
+  if(NOT arch_hint)
+    set(arch_hint "x86")
+  endif()
+
+  # default env in a 64-bit app on Win64:
+  # ProgramFiles=C:\Program Files
+  # ProgramFiles(x86)=C:\Program Files (x86)
+  # ProgramW6432=C:\Program Files
+  #
+  # default env in a 32-bit app on Win64:
+  # ProgramFiles=C:\Program Files (x86)
+  # ProgramFiles(x86)=C:\Program Files (x86)
+  # ProgramW6432=C:\Program Files
+  #
+  # default env in a 32-bit app on Win32:
+  # ProgramFiles=C:\Program Files
+  # ProgramFiles(x86) NOT DEFINED
+  # ProgramW6432 NOT DEFINED
+
+  # By default, use the ProgramFiles env var as the base value of
+  # CMAKE_INSTALL_PREFIX:
+  #
+  set(_PREFIX_ENV_VAR "ProgramFiles")
+
+  if ("$ENV{ProgramW6432}" STREQUAL "")
+    # running on 32-bit Windows
+    # must be a 32-bit CMake, too...
+    #message("guess: this is a 32-bit CMake running on 32-bit Windows")
+  else()
+    # running on 64-bit Windows
+    if ("$ENV{ProgramW6432}" STREQUAL "$ENV{ProgramFiles}")
+      # 64-bit CMake
+      #message("guess: this is a 64-bit CMake running on 64-bit Windows")
+      if(NOT "${arch_hint}" STREQUAL "x64")
+      # building 32-bit targets
+        set(_PREFIX_ENV_VAR "ProgramFiles(x86)")
+      endif()
+    else()
+      # 32-bit CMake
+      #message("guess: this is a 32-bit CMake running on 64-bit Windows")
+      if("${arch_hint}" STREQUAL "x64")
+      # building 64-bit targets
+        set(_PREFIX_ENV_VAR "ProgramW6432")
+      endif()
+    endif()
+  endif()
+
+  #if("${arch_hint}" STREQUAL "x64")
+  #  message("guess: you are building a 64-bit app")
+  #else()
+  #  message("guess: you are building a 32-bit app")
+  #endif()
+
+  if(NOT "$ENV{${_PREFIX_ENV_VAR}}" STREQUAL "")
+    file(TO_CMAKE_PATH "$ENV{${_PREFIX_ENV_VAR}}" _base)
+  elseif(NOT "$ENV{SystemDrive}" STREQUAL "")
+    set(_base "$ENV{SystemDrive}/Program Files")
+  else()
+    set(_base "C:/Program Files")
+  endif()
+
+  set(${var} "${_base}" PARENT_SCOPE)
+endfunction()
+
+
 # Set a variable to indicate whether the value of CMAKE_INSTALL_PREFIX
 # was initialized by the block below.  This is useful for user
 # projects to change the default prefix while still allowing the
@@ -65,23 +153,11 @@ IF(CMAKE_HOST_UNIX)
   SET(CMAKE_INSTALL_PREFIX "/usr/local"
     CACHE PATH "Install path prefix, prepended onto install directories.")
 ELSE(CMAKE_HOST_UNIX)
-  IF("$ENV{ProgramFiles}" MATCHES "^$")
-    IF("$ENV{SystemDrive}" MATCHES "^$")
-      SET(CMAKE_GENERIC_PROGRAM_FILES "C:/Program Files")
-    ELSE("$ENV{SystemDrive}" MATCHES "^$")
-      SET(CMAKE_GENERIC_PROGRAM_FILES "$ENV{SystemDrive}/Program Files")
-    ENDIF("$ENV{SystemDrive}" MATCHES "^$")
-  ELSE("$ENV{ProgramFiles}" MATCHES "^$")
-    SET(CMAKE_GENERIC_PROGRAM_FILES "$ENV{ProgramFiles}")
-  ENDIF("$ENV{ProgramFiles}" MATCHES "^$")
+  GetDefaultWindowsPrefixBase(CMAKE_GENERIC_PROGRAM_FILES)
   SET(CMAKE_INSTALL_PREFIX
     "${CMAKE_GENERIC_PROGRAM_FILES}/${PROJECT_NAME}"
     CACHE PATH "Install path prefix, prepended onto install directories.")
   SET(CMAKE_GENERIC_PROGRAM_FILES)
-
-  # Make sure the prefix uses forward slashes.
-  STRING(REGEX REPLACE "\\\\" "/"
-    CMAKE_INSTALL_PREFIX "${CMAKE_INSTALL_PREFIX}")
 ENDIF(CMAKE_HOST_UNIX)
 
 MARK_AS_ADVANCED(