diff --git a/README.md b/README.md
index 7ab91e4372..bf5e918921 100644
--- a/README.md
+++ b/README.md
@@ -55,8 +55,8 @@ Windows
 You can build Panda3D with the Microsoft Visual C++ 2015, 2017 or 2019 compiler,
 which can be downloaded for free from the [Visual Studio site](https://visualstudio.microsoft.com/downloads/).
 You will also need to install the [Windows 10 SDK](https://developer.microsoft.com/en-us/windows/downloads/windows-10-sdk),
-and if you intend to target Windows XP, you will also need the Windows 7.1A
-SDK (which can be installed from the Visual Studio Installer).
+and if you intend to target Windows Vista, you will also need the
+[Windows 8.1 SDK](https://go.microsoft.com/fwlink/p/?LinkId=323507).
 
 You will also need to have the third-party dependency libraries available for
 the build scripts to use.  These are available from one of these two URLs,
@@ -70,8 +70,8 @@ building them from source.
 After acquiring these dependencies, you can build Panda3D from the command
 prompt using the following command.  Change the `--msvc-version` option based
 on your version of Visual C++; 2019 is 14.2, 2017 is 14.1, and 2015 is 14.
-Remove the `--windows-sdk=10` option if you need to support Windows XP, which
-requires the Windows 7.1A SDK.
+Remove the `--windows-sdk=10` option if you need to support Windows Vista,
+which requires the Windows 8.1 SDK.
 
 ```bash
 makepanda\makepanda.bat --everything --installer --msvc-version=14.2 --windows-sdk=10 --no-eigen --threads=2
diff --git a/makepanda/makepanda.py b/makepanda/makepanda.py
index 2daec23e7a..911392b2b7 100755
--- a/makepanda/makepanda.py
+++ b/makepanda/makepanda.py
@@ -146,7 +146,7 @@ def usage(problem):
     print("  --nothing         (disable every third-party lib)")
     print("  --everything      (enable every third-party lib)")
     print("  --directx-sdk=X   (specify version of DirectX SDK to use: jun2010, aug2009, mar2009, aug2006)")
-    print("  --windows-sdk=X   (specify Windows SDK version, eg. 7.0, 7.1 or 10.  Default is 7.1)")
+    print("  --windows-sdk=X   (specify Windows SDK version, eg. 7.1, 8.1 or 10.  Default is 8.1)")
     print("  --msvc-version=X  (specify Visual C++ version, eg. 10, 11, 12, 14, 14.1, 14.2.  Default is 14)")
     print("  --use-icl         (experimental setting to use an intel compiler instead of MSVC on Windows)")
     print("")
@@ -321,10 +321,6 @@ def parseopts(args):
             time.sleep(1.0)
             sys.exit(1)
 
-        if not WINDOWS_SDK:
-            print("No Windows SDK version specified. Defaulting to '7.1'.")
-            WINDOWS_SDK = '7.1'
-
     if clean_build and os.path.isdir(GetOutputDir()):
         print("Deleting %s" % (GetOutputDir()))
         shutil.rmtree(GetOutputDir())
@@ -1075,10 +1071,6 @@ def CompileCxx(obj,src,opts):
             if GetTargetArch() == 'x64':
                 cmd += " /DWIN64_VC /DWIN64"
 
-            if WINDOWS_SDK.startswith('7.') and MSVC_VERSION > (10,):
-                # To preserve Windows XP compatibility.
-                cmd += " /D_USING_V110_SDK71_"
-
             cmd += " /W3 " + BracketNameWithQuotes(src)
             oscmd(cmd)
         else:
diff --git a/makepanda/makepandacore.py b/makepanda/makepandacore.py
index fb31536fad..a5c14d1d7d 100644
--- a/makepanda/makepandacore.py
+++ b/makepanda/makepandacore.py
@@ -2289,16 +2289,17 @@ def SdkLocateVisualStudio(version=(10,0)):
 
     print("Using MSVC %s" % version_str)
 
-def SdkLocateWindows(version = '7.1'):
+def SdkLocateWindows(version=None):
     if GetTarget() != "windows" or GetHost() != "windows":
         return
 
-    version = version.upper()
+    if version:
+        version = version.upper()
 
     if version == '10':
         version = '10.0'
 
-    if version.startswith('10.') and version.count('.') == 1:
+    if version and version.startswith('10.') and version.count('.') == 1:
         # Choose the latest version of the Windows 10 SDK.
         platsdk = GetRegistryKey("SOFTWARE\\Microsoft\\Windows Kits\\Installed Roots", "KitsRoot10")
 
@@ -2333,7 +2334,7 @@ def SdkLocateWindows(version = '7.1'):
                 # No suitable version found.
                 platsdk = None
 
-    elif version.startswith('10.'):
+    elif version and version.startswith('10.'):
         # We chose a specific version of the Windows 10 SDK.  Verify it exists.
         platsdk = GetRegistryKey("SOFTWARE\\Microsoft\\Windows Kits\\Installed Roots", "KitsRoot10")
 
@@ -2347,19 +2348,27 @@ def SdkLocateWindows(version = '7.1'):
         if platsdk and not os.path.isdir(os.path.join(platsdk, 'Include', version)):
             platsdk = None
 
-    elif version == '8.1':
+    elif version == '8.1' or not version:
         platsdk = GetRegistryKey("SOFTWARE\\Microsoft\\Windows Kits\\Installed Roots", "KitsRoot81")
 
         # Fallback in case we can't read the registry.
         if not platsdk or not os.path.isdir(platsdk):
             platsdk = "C:\\Program Files (x86)\\Windows Kits\\8.1\\"
 
+        if not version:
+            if not os.path.isdir(platsdk):
+                # Fall back to 7.1 SDK.
+                return SdkLocateWindows("7.1")
+            version = '8.1'
+
     elif version == '8.0':
         platsdk = GetRegistryKey("SOFTWARE\\Microsoft\\Windows Kits\\Installed Roots", "KitsRoot")
 
     else:
         platsdk = GetRegistryKey("SOFTWARE\\Microsoft\\Microsoft SDKs\\Windows\\v" + version, "InstallationFolder")
 
+        DefSymbol("ALWAYS", "_USING_V110_SDK71_")
+
         if not platsdk or not os.path.isdir(platsdk):
             # Most common location.  Worth a try.
             platsdk = GetProgramFiles() + "\\Microsoft SDKs\\Windows\\v" + version