diff --git a/src/calibre/ebooks/conversion/plumber.py b/src/calibre/ebooks/conversion/plumber.py
index 11e434c635..6ce1b42356 100644
--- a/src/calibre/ebooks/conversion/plumber.py
+++ b/src/calibre/ebooks/conversion/plumber.py
@@ -679,7 +679,7 @@ OptionRecommendation(name='search_replace',
         self.archive_input_tdir = None
         if input_fmt in ARCHIVE_FMTS:
             self.log('Processing archive...')
-            tdir = PersistentTemporaryDirectory('_plumber_archive')
+            tdir = PersistentTemporaryDirectory('_pl_arc')
             self.input, input_fmt = self.unarchive(self.input, tdir)
             self.archive_input_tdir = tdir
         if os.access(self.input, os.R_OK):
diff --git a/src/calibre/ptempfile.py b/src/calibre/ptempfile.py
index 706a96b4b6..96271fbeaf 100644
--- a/src/calibre/ptempfile.py
+++ b/src/calibre/ptempfile.py
@@ -30,6 +30,8 @@ def remove_dir(x):
         pass
 
 def app_prefix(prefix):
+    if iswindows:
+        return '%s_'%__appname__
     return '%s_%s_%s'%(__appname__, __version__, prefix)
 
 def base_dir():