diff --git a/pipenv/core.py b/pipenv/core.py
index b059ddc9..38cb5b7b 100644
--- a/pipenv/core.py
+++ b/pipenv/core.py
@@ -315,17 +315,24 @@ def ensure_pipfile(validate=True, skip_requirements=False, system=False):
 
 
 def find_a_system_python(python):
+    from .vendor.pythonfinder import Finder
+    # system always refers to sys.executable, which could point at a virtualenv
+    # for global searches we most likely want to turn that off
+    finder = Finder(system=False, global_search=True)
     if not python:
         return None
+    # when using the python launcher on windows we can find the versions ourselves
+    if os.name == 'nt' and python.startswith("py -"):
+        python = python[len("py -"):]
     if python.startswith("py"):
-        return system_which(python)
+        python_entry = finder.which(python)
+        if python_entry:
+            return python_entry.path.as_posix()
     if os.path.isabs(python):
         return python
-    from .vendor import pythonfinder
-    finder = pythonfinder.Finder()
     python_entry = finder.find_python_version(python)
     if python_entry:
-        return str(python_entry.path)
+        return python_entry.path.as_posix()
     return None