diff --git a/lib/librarian/posix.rb b/lib/librarian/posix.rb
index 6ef6ab83..4034ddf9 100644
--- a/lib/librarian/posix.rb
+++ b/lib/librarian/posix.rb
@@ -1,10 +1,20 @@
 require "open3"
+require "rbconfig"
 
 require "librarian/error"
 
 module Librarian
   module Posix
 
+    module Platform
+      extend self
+
+      def win?
+        host_os = RbConfig::CONFIG["host_os"].dup.freeze
+        host_os =~ /mswin|mingw/
+      end
+    end
+
     class << self
 
       # Cross-platform way of finding an executable in the $PATH.
@@ -119,7 +129,9 @@ def run_spawn!(command, options = { })
         [i, o, e].flatten(1).each{|io| io.close unless io.closed?}
       end
 
-      if defined?(JRuby) # built with jruby-1.7.9 in mind
+      # jruby-1.7.9 can't fork and doesn't have a decent spawn
+      # windows can't fork and doesn't have a decent spawn
+      if defined?(JRuby) || Platform.win?
 
         alias_method :run!, :run_popen3!