Index: pathname.rb
===================================================================
--- pathname.rb	(revision 4471)
+++ pathname.rb	(working copy)
@@ -285,6 +285,8 @@
   def prepend_prefix(prefix, relpath)
     if relpath.empty?
       File.dirname(prefix)
+    elsif prefix =~ /^[a-zA-Z]:$/
+      File.join(prefix,relpath)
     elsif /#{SEPARATOR_PAT}/ =~ prefix
       prefix = File.dirname(prefix)
       prefix = File.join(prefix, "") if File.basename(prefix + 'a') != 'a'
@@ -408,6 +410,8 @@
         next
       elsif n == '..'
         resolved.pop
+      elsif n =~ /^[a-zA-Z]:$/
+        resolved << n
       else
         path = prepend_prefix(prefix, File.join(*(resolved + [n])))
         if h.include? path
@@ -446,7 +450,9 @@
   def realpath
     path = @path
     prefix, names = split_names(path)
-    if prefix == ''
+    if prefix == '' && names[0] =~ /^[a-zA-Z]:$/
+      prefix = names.shift
+    elsif prefix == ''
       prefix, names2 = split_names(Dir.pwd)
       names = names2 + names
     end