diff --git a/src/BizHawk.Client.EmuHawk/MainForm.cs b/src/BizHawk.Client.EmuHawk/MainForm.cs
index d2fc8e4b65..4466588cca 100644
--- a/src/BizHawk.Client.EmuHawk/MainForm.cs
+++ b/src/BizHawk.Client.EmuHawk/MainForm.cs
@@ -3,7 +3,6 @@ using System.Collections.Generic;
 using System.Diagnostics;
 using System.Drawing;
 using System.Drawing.Imaging;
-using System.Globalization;
 using System.IO;
 using System.Linq;
 using System.Reflection;
@@ -29,6 +28,7 @@ using BizHawk.Client.EmuHawk.ToolExtensions;
 using BizHawk.Client.EmuHawk.CoreExtensions;
 using BizHawk.Client.EmuHawk.CustomControls;
 using BizHawk.Common.PathExtensions;
+using BizHawk.Common.StringExtensions;
 using BizHawk.Emulation.Common.Base_Implementations;
 using BizHawk.Emulation.Cores.Consoles.NEC.PCE;
 using BizHawk.Emulation.Cores.Nintendo.SNES9X;
@@ -628,30 +628,21 @@ namespace BizHawk.Client.EmuHawk
 
 			if (!OSTailoredCode.IsUnixHost && !Config.SkipOutdatedOsCheck)
 			{
-				static string GetRegValue(string key)
+				var (winVersion, win10Release) = OSTailoredCode.HostWindowsVersion.Value;
+				var message = winVersion switch
 				{
-					using var proc = OSTailoredCode.ConstructSubshell("REG", $@"QUERY ""HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion"" /V {key}");
-					proc.Start();
-					return proc.StandardOutput.ReadToEnd().Split(new[] { "\r\n", "\n" }, StringSplitOptions.RemoveEmptyEntries)[1].Split(new[] { '\t', ' ' }, StringSplitOptions.RemoveEmptyEntries)[2];
-				}
-				var winVer = float.Parse(GetRegValue("CurrentVersion"), NumberFormatInfo.InvariantInfo);
-				if (winVer < 6.3f)
+					OSTailoredCode.WindowsVersion._10 when win10Release < 1809 => $"Quick reminder: version {win10Release} of Windows 10 is no longer supported by Microsoft. EmuHawk will continue to work, but please update to at least 1809 \"Redstone 5\" for increased security.",
+					OSTailoredCode.WindowsVersion._10 => null,
+					OSTailoredCode.WindowsVersion._8_1 => null, // still CBB
+					_ => $"Quick reminder: Windows {winVersion.ToString().RemovePrefix('_').Replace("_", ".")} is no longer supported by Microsoft. EmuHawk will continue to work, but please get a new operating system for increased security (either Windows 8.1, Windows 10, or a GNU+Linux distro)."
+				};
+#if false
+				if (message != null)
 				{
-					// less than is just easier than equals
-					string message = ($"Quick reminder: Windows {(winVer < 6.2f ? winVer < 6.1f ? winVer < 6.0f ? "XP" : "Vista" : "7" : "8")} is no longer supported by Microsoft. EmuHawk will continue to work, but please get a new operating system for increased security (either Windows 8.1, Windows 10, or a GNU+Linux distro).");
-				}
-				else if (GetRegValue("ProductName").Contains("Windows 10"))
-				{
-					var win10version = int.Parse(GetRegValue("ReleaseId"));
-					if (win10version < 1809)
-					{
-						string message = ($"Quick reminder: version {win10version} of Windows 10 is no longer supported by Microsoft. EmuHawk will continue to work, but please update to at least 1809 \"Redstone 5\" for increased security.");
-					}
-				}
-				else
-				{
-					// 8.1: can't be bothered writing code for KB installed check, not that I have a Win8.1 machine to test on anyway, so it gets a free pass --yoshi
+					using var box = new ExceptionBox(message);
+					box.ShowDialog();
 				}
+#endif
 			}
 		}
 
diff --git a/src/BizHawk.Common/Extensions/StringExtensions.cs b/src/BizHawk.Common/Extensions/StringExtensions.cs
index deb8b54bba..b2105959b9 100644
--- a/src/BizHawk.Common/Extensions/StringExtensions.cs
+++ b/src/BizHawk.Common/Extensions/StringExtensions.cs
@@ -11,6 +11,18 @@ namespace BizHawk.Common.StringExtensions
 		public static bool In(this string str, params string[] options) =>
 			options.Any(opt => string.Equals(opt, str, StringComparison.InvariantCultureIgnoreCase));
 
+		/// <returns>
+		/// <paramref name="str"/> with the first char removed, or
+		/// the original <paramref name="str"/> if the first char of <paramref name="str"/> is not <paramref name="prefix"/>
+		/// </returns>
+		public static string RemovePrefix(this string str, char prefix) => str.RemovePrefix(prefix, notFoundValue: str);
+
+		/// <returns>
+		/// <paramref name="str"/> with the first char removed, or
+		/// <paramref name="notFoundValue"/> if the first char of <paramref name="str"/> is not <paramref name="prefix"/>
+		/// </returns>
+		public static string RemovePrefix(this string str, char prefix, string notFoundValue) => str.Length != 0 && str[0] == prefix ? str.Substring(1, str.Length - 1) : notFoundValue;
+
 		/// <returns>
 		/// <paramref name="str"/> with the last char removed, or
 		/// the original <paramref name="str"/> if the last char of <paramref name="str"/> is not <paramref name="suffix"/>
diff --git a/src/BizHawk.Common/OSTailoredCode.cs b/src/BizHawk.Common/OSTailoredCode.cs
index c3b14dd498..da5e85b14e 100644
--- a/src/BizHawk.Common/OSTailoredCode.cs
+++ b/src/BizHawk.Common/OSTailoredCode.cs
@@ -1,5 +1,6 @@
 using System;
 using System.Diagnostics;
+using System.Globalization;
 using System.Runtime.InteropServices;
 
 #if EXE_PROJECT
@@ -15,6 +16,36 @@ namespace BizHawk.Common
 			? SimpleSubshell("uname", "-s", "Can't determine OS") == "Darwin" ? DistinctOS.macOS : DistinctOS.Linux
 			: DistinctOS.Windows;
 
+		private static readonly Lazy<(WindowsVersion, int?)?> _HostWindowsVersion = new Lazy<(WindowsVersion, int?)?>(() =>
+		{
+			static string GetRegValue(string key)
+			{
+				using var proc = ConstructSubshell("REG", $@"QUERY ""HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion"" /V {key}");
+				proc.Start();
+				return proc.StandardOutput.ReadToEnd().Split(new[] { "\r\n", "\n" }, StringSplitOptions.RemoveEmptyEntries)[1].Split(new[] { '\t', ' ' }, StringSplitOptions.RemoveEmptyEntries)[2];
+			}
+			if (CurrentOS != DistinctOS.Windows) return null;
+			var rawWinVer = float.Parse(GetRegValue("CurrentVersion"), NumberFormatInfo.InvariantInfo); // contains '.' even when system-wide decimal separator is ','
+			WindowsVersion winVer; // sorry if this elif chain is confusing, I couldn't be bothered writing and testing float equality --yoshi
+			if (rawWinVer < 6.0f) winVer = WindowsVersion.XP;
+			else if (rawWinVer < 6.1f) winVer = WindowsVersion.Vista;
+			else if (rawWinVer < 6.2f) winVer = WindowsVersion._7;
+			else if (rawWinVer < 6.3f) winVer = WindowsVersion._8;
+			else
+			{
+				// 8.1 and 10 are both version 6.3
+				if (GetRegValue("ProductName").Contains("Windows 10"))
+				{
+					return (WindowsVersion._10, int.Parse(GetRegValue("ReleaseId")));
+				}
+				// ...else we're on 8.1. Can't be bothered writing code for KB installed check, not that I have a Win8.1 machine to test on anyway, so it gets a free pass --yoshi
+				winVer = WindowsVersion._8_1;
+			}
+			return (winVer, null);
+		});
+
+		public static (WindowsVersion Version, int? Win10Release)? HostWindowsVersion => _HostWindowsVersion.Value;
+
 		public static readonly bool IsUnixHost = CurrentOS != DistinctOS.Windows;
 
 		private static readonly Lazy<ILinkedLibManager> _LinkedLibManager = new Lazy<ILinkedLibManager>(() => CurrentOS switch
@@ -123,6 +154,16 @@ namespace BizHawk.Common
 			Windows
 		}
 
+		public enum WindowsVersion
+		{
+			XP,
+			Vista,
+			_7,
+			_8,
+			_8_1,
+			_10
+		}
+
 		/// <param name="cmd">POSIX <c>$0</c></param>
 		/// <param name="args">POSIX <c>$*</c> (space-delimited)</param>
 		/// <param name="checkStdout">stdout is discarded if false</param>
diff --git a/src/BizHawk.Tests/Common/StringExtensions/StringExtensionTests.cs b/src/BizHawk.Tests/Common/StringExtensions/StringExtensionTests.cs
index ba261011cb..faf9ea2d00 100644
--- a/src/BizHawk.Tests/Common/StringExtensions/StringExtensionTests.cs
+++ b/src/BizHawk.Tests/Common/StringExtensions/StringExtensionTests.cs
@@ -17,5 +17,18 @@ namespace BizHawk.Tests.Common.StringExtensions
 			var actual = "hello world".In(strArray);
 			Assert.IsTrue(actual);
 		}
+
+		[TestMethod]
+		public void TestRemovePrefix()
+		{
+			const string abcdef = "abcdef";
+			const string qrs = "qrs";
+
+			Assert.AreEqual("bcdef", abcdef.RemovePrefix('a', qrs));
+			Assert.AreEqual(string.Empty, "a".RemovePrefix('a', qrs));
+			Assert.AreEqual(qrs, abcdef.RemovePrefix('c', qrs));
+			Assert.AreEqual(qrs, abcdef.RemovePrefix('x', qrs));
+			Assert.AreEqual(qrs, string.Empty.RemovePrefix('a', qrs));
+		}
 	}
 }