diff --git a/Buildscr b/Buildscr
index 08be3ec6..12ff2fcf 100644
--- a/Buildscr
+++ b/Buildscr
@@ -199,8 +199,13 @@ ifneq "$(cross_winsigncode)" "" in putty/windows do $(cross_winsigncode) -N -i h
 # Build a WiX MSI installer, for each of build32 and build64.
 in putty/windows with wixonlinux do candle -arch x86 -dRealPlatform=x86 -dDllOk=yes -dBuilddir=build32/ -dWinver="$(Winver)" -dPuttytextver="$(Puttytextver)" installer.wxs && light -ext WixUIExtension -ext WixUtilExtension -sval installer.wixobj -o installer32.msi -spdb
 in putty/windows with wixonlinux do candle -arch x64 -dRealPlatform=x64 -dDllOk=yes -dBuilddir=build64/ -dWinver="$(Winver)" -dPuttytextver="$(Puttytextver)" installer.wxs && light -ext WixUIExtension -ext WixUtilExtension -sval installer.wixobj -o installer64.msi -spdb
-in putty/windows with wixonlinux do candle -arch x86 -dRealPlatform=Arm -dDllOk=no -dBuilddir=abuild32/ -dWinver="$(Winver)" -dPuttytextver="$(Puttytextver)" installer.wxs && light -ext WixUIExtension -ext WixUtilExtension -sval installer.wixobj -o installera32.msi -spdb
-in putty/windows with wixonlinux do candle -arch x86 -dRealPlatform=Arm64 -dDllOk=no -dBuilddir=abuild64/ -dWinver="$(Winver)" -dPuttytextver="$(Puttytextver)" installer.wxs && light -ext WixUIExtension -ext WixUtilExtension -sval installer.wixobj -o installera64.msi -spdb
+in putty/windows with wixonlinux do candle -arch x64 -dRealPlatform=Arm -dDllOk=no -dBuilddir=abuild32/ -dWinver="$(Winver)" -dPuttytextver="$(Puttytextver)" installer.wxs && light -ext WixUIExtension -ext WixUtilExtension -sval installer.wixobj -o installera32.msi -spdb
+in putty/windows with wixonlinux do candle -arch x64 -dRealPlatform=Arm64 -dDllOk=no -dBuilddir=abuild64/ -dWinver="$(Winver)" -dPuttytextver="$(Puttytextver)" installer.wxs && light -ext WixUIExtension -ext WixUtilExtension -sval installer.wixobj -o installera64.msi -spdb
+
+# Bodge the platform fields for the Windows on Arm installers, since
+# WiX 3 doesn't understand Arm platform names itself.
+in putty/windows do ./msiplatform.py installera32.msi Arm
+in putty/windows do ./msiplatform.py installera64.msi Arm64
 
 # Sign the Windows installers.
 ifneq "$(cross_winsigncode)" "" in putty/windows do $(cross_winsigncode) -i https://www.chiark.greenend.org.uk/~sgtatham/putty/ -n "PuTTY Installer" installer32.msi installer64.msi installera32.msi installera64.msi
diff --git a/windows/installer.wxs b/windows/installer.wxs
index 47da2c9e..005d7e03 100644
--- a/windows/installer.wxs
+++ b/windows/installer.wxs
@@ -5,17 +5,20 @@
 <?if $(var.RealPlatform) = x64 ?>
   <?define Bitness = " (64-bit)" ?>
   <?define RegKeyPathLocation = "Software\SimonTatham\PuTTY64" ?>
-  <?define PlatformProgramFilesFolder = "ProgramFiles64Folder" ?>
 <?else ?>
   <?define Bitness = "" ?>
   <?define RegKeyPathLocation = "Software\SimonTatham\PuTTY" ?>
-  <?define PlatformProgramFilesFolder = "ProgramFilesFolder" ?>
 <?endif ?>
 
 <?if $(var.RealPlatform) = x86 ?>
   <?define InstallerVersion = "100" ?>
-<?else ?>
+  <?define PlatformProgramFilesFolder = "ProgramFilesFolder" ?>
+<?elseif $(var.RealPlatform) = x64 ?>
   <?define InstallerVersion = "200" ?>
+  <?define PlatformProgramFilesFolder = "ProgramFiles64Folder" ?>
+<?else ?>
+  <?define InstallerVersion = "500" ?>
+  <?define PlatformProgramFilesFolder = "ProgramFiles64Folder" ?>
 <?endif ?>
 
 <?if $(var.RealPlatform) = x64 ?>
diff --git a/windows/msiplatform.py b/windows/msiplatform.py
new file mode 100755
index 00000000..eea4272f
--- /dev/null
+++ b/windows/msiplatform.py
@@ -0,0 +1,63 @@
+#!/usr/bin/env python
+
+import argparse
+import os
+import tempfile
+import shutil
+import subprocess
+import pipes
+
+def run(command, verbose):
+    if verbose:
+        sys.stdout.write("$ {}\n".format(" ".join(
+            pipes.quote(word) for word in command)))
+    out = subprocess.check_output(command)
+    if verbose:
+        sys.stdout.write("".join(
+            "> {}\n".format(line) for line in out.splitlines()))
+
+def set_platform(msi, platform, verbose):
+    run(["msidump", "-t", msi], verbose)
+
+    summary_stream = "_SummaryInformation.idt"
+
+    with open(summary_stream) as fh:
+        lines = [line.rstrip("\r\n").split("\t")
+                 for line in iter(fh.readline, "")]
+
+    for line in lines[3:]:
+        if line[0] == "7":
+            line[1] = ";".join([platform] + line[1].split(";", 1)[1:])
+
+    with open(summary_stream, "w") as fh:
+        for line in lines:
+            fh.write("\t".join(line) + "\r\n")
+
+    run(["msibuild", msi, "-i", summary_stream], verbose)
+
+def main():
+    parser = argparse.ArgumentParser(
+        description='Change the platform field of an MSI installer package.')
+    parser.add_argument("msi", help="MSI installer file.")
+    parser.add_argument("platform", help="New value for the platform field.")
+    parser.add_argument("-v", "--verbose", action="store_true",
+                        help="Log what this script is doing.")
+    parser.add_argument("-k", "--keep", action="store_true",
+                        help="Don't delete the temporary working directory.")
+    args = parser.parse_args()
+
+    msi = os.path.abspath(args.msi)
+    msidir = os.path.dirname(msi)
+    try:
+        tempdir = tempfile.mkdtemp(dir=msidir)
+        os.chdir(tempdir)
+        set_platform(msi, args.platform, args.verbose)
+    finally:
+        if args.keep:
+            sys.stdout.write(
+                "Retained temporary directory {}\n".format(tempdir))
+        else:
+            shutil.rmtree(tempdir)
+
+if __name__ == '__main__':
+    main()