diff --git a/launcher/build.py b/launcher/build.py
index eca68b3d..b3fae563 100644
--- a/launcher/build.py
+++ b/launcher/build.py
@@ -14,6 +14,11 @@ RUN = ('import os, runpy, sys; p = sys.argv[1]; s = sys.argv[2]; sys.argv = sys.
        'runpy.run_path(s, run_name="__main__")')
 CREATE_NO_WINDOW = 0x08000000
 CREATE_NEW_PROCESS_GROUP = 0x00000200
+# The build runs below normal priority: a compile on every processor for ten minutes and more otherwise takes the
+# PC from whatever the player does meanwhile, which the job screen invites ("Keep using your PC if you like"). On an
+# idle PC it costs nothing. Windows hands a below-normal process's class on to the processes it starts, so the
+# compilers, the lifter's workers and the shader preparation that make_game.py starts run that way too.
+BELOW_NORMAL_PRIORITY_CLASS = 0x00004000
 
 
 def command(layout, args):
@@ -61,7 +66,7 @@ class Run:
         self.error = None
         kw = {}
         if os.name == 'nt':
-            kw['creationflags'] = CREATE_NO_WINDOW | CREATE_NEW_PROCESS_GROUP
+            kw['creationflags'] = CREATE_NO_WINDOW | CREATE_NEW_PROCESS_GROUP | BELOW_NORMAL_PRIORITY_CLASS
         else:
             kw['start_new_session'] = True
         env = dict(os.environ, PYTHONUNBUFFERED='1', PYTHONIOENCODING='utf-8')
diff --git a/tests/launcher_test.py b/tests/launcher_test.py
index f0069fcf..73d167a5 100644
--- a/tests/launcher_test.py
+++ b/tests/launcher_test.py
@@ -2743,6 +2743,24 @@ class JobTest(Tmp):
             App.job_finished(a)
         self.assertEqual(a.message, 'The game is built. Select Play.')
 
+    @unittest.skipUnless(os.name == 'nt', 'Windows priority classes')
+    def test_the_build_and_what_it_starts_run_below_normal_priority(self):
+        """build.Run starts the build below normal priority, and the processes the build starts (the compilers)
+        get that class from it, so a build on every processor leaves the PC usable."""
+        script = self.tmp / 'priority.py'
+        script.write_text(
+            'import ctypes, subprocess, sys\n'
+            'k = ctypes.windll.kernel32\n'
+            'k.GetCurrentProcess.restype = ctypes.c_void_p\n'
+            'k.GetPriorityClass.argtypes = [ctypes.c_void_p]\n'
+            'print(sys.argv[1], hex(k.GetPriorityClass(k.GetCurrentProcess())), flush=True)\n'
+            'if sys.argv[1] == "build":\n'
+            '    subprocess.run([sys.executable, __file__, "compiler"])\n')
+        lines = []
+        run = build.Run([sys.executable, str(script), 'build'], lambda ev: None, lines.append)
+        self.assertEqual(run.wait(), 0, lines)
+        self.assertEqual(lines, ['build 0x4000', 'compiler 0x4000'])   # BELOW_NORMAL_PRIORITY_CLASS
+
     def test_an_install_goes_on_from_the_unpacked_game_files(self):
         """Begin Installation into a folder whose game files an earlier run unpacked builds on from them (--rebuild
         for this processor's level), so a moved or deleted dump is not asked for; without them it reads the dump."""