# Set-ExplorerSeparateProcess-AllUsers.ps1
# Enables: "Launch folder windows in a separate process" for ALL existing users + Default User
# Sets: HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced\SeparateProcess = 1

[CmdletBinding()]
param()

function Assert-Admin {
    $id  = [Security.Principal.WindowsIdentity]::GetCurrent()
    $pri = New-Object Security.Principal.WindowsPrincipal($id)
    if (-not $pri.IsInRole([Security.Principal.WindowsBuiltInRole]::Administrator)) {
        throw "This script must be run as Administrator."
    }
}

function Set-SeparateProcessInHivePath {
    param(
        [Parameter(Mandatory=$true)][string]$HiveRoot # e.g. "Registry::HKEY_USERS\S-1-5-21-...\"
    )
    $advPath = Join-Path $HiveRoot "Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced"
    New-Item -Path $advPath -Force | Out-Null
    New-ItemProperty -Path $advPath -Name "SeparateProcess" -Value 1 -PropertyType DWORD -Force | Out-Null
}

function Load-Hive {
    param(
        [Parameter(Mandatory=$true)][string]$MountName,   # e.g. "TempHive_1234"
        [Parameter(Mandatory=$true)][string]$NtUserDatPath
    )
    $mount = "HKU\$MountName"
    $null = & reg.exe load $mount $NtUserDatPath 2>$null
    return $mount
}

function Unload-Hive {
    param([Parameter(Mandatory=$true)][string]$Mount) # e.g. "HKU\TempHive_1234"
    $null = & reg.exe unload $Mount 2>$null
}

Assert-Admin

Write-Host "Applying Explorer setting to all users + Default User..." -ForegroundColor Cyan

# 1) Update all existing user profiles (even if not logged in)
$profileRoots = Get-ItemProperty "HKLM:\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList\*" |
    Where-Object {
        $_.PSChildName -match '^S-1-5-21-' -and
        $_.ProfileImagePath -and
        (Test-Path $_.ProfileImagePath)
    } |
    Select-Object PSChildName, ProfileImagePath

$updated = 0
$skipped = 0
$failed  = 0

foreach ($p in $profileRoots) {
    $sid   = $p.PSChildName
    $path  = $p.ProfileImagePath
    $ntDat = Join-Path $path "NTUSER.DAT"

    # Skip special/system-ish profiles by path
    if ($path -match '\\(Default|Default User|Public|All Users)$') { $skipped++; continue }
    if (-not (Test-Path $ntDat)) { $skipped++; continue }

    # If hive is already loaded (user logged in), just set it directly
    $loadedHive = "Registry::HKEY_USERS\$sid"
    if (Test-Path $loadedHive) {
        try {
            Set-SeparateProcessInHivePath -HiveRoot $loadedHive
            $updated++
            Write-Host "OK   (loaded)  $sid  [$path]"
        } catch {
            $failed++
            Write-Warning "FAIL (loaded)  $sid  [$path]  :: $($_.Exception.Message)"
        }
        continue
    }

    # Otherwise load NTUSER.DAT temporarily, modify, unload
    $mountName = "TempHive_{0}" -f ([Guid]::NewGuid().ToString("N"))
    $mount = $null
    try {
        $mount = Load-Hive -MountName $mountName -NtUserDatPath $ntDat
        $hiveRoot = "Registry::HKEY_USERS\$mountName"
        Set-SeparateProcessInHivePath -HiveRoot $hiveRoot
        $updated++
        Write-Host "OK   (loaded*) $sid  [$path]"
    } catch {
        $failed++
        Write-Warning "FAIL          $sid  [$path]  :: $($_.Exception.Message)"
    } finally {
        if ($mount) { Unload-Hive -Mount $mount }
    }
}

# 2) Update Default User so NEW profiles inherit it
$defaultNtDat = "C:\Users\Default\NTUSER.DAT"
if (Test-Path $defaultNtDat) {
    $mountName = "TempDefault_{0}" -f ([Guid]::NewGuid().ToString("N"))
    $mount = $null
    try {
        $mount = Load-Hive -MountName $mountName -NtUserDatPath $defaultNtDat
        $hiveRoot = "Registry::HKEY_USERS\$mountName"
        Set-SeparateProcessInHivePath -HiveRoot $hiveRoot
        Write-Host "OK   Default User updated [$defaultNtDat]"
    } catch {
        Write-Warning "FAIL Default User [$defaultNtDat] :: $($_.Exception.Message)"
    } finally {
        if ($mount) { Unload-Hive -Mount $mount }
    }
} else {
    Write-Warning "Default User NTUSER.DAT not found at: $defaultNtDat"
}

Write-Host ""
Write-Host "Done." -ForegroundColor Green
Write-Host "Updated: $updated   Skipped: $skipped   Failed: $failed"
Write-Host ""
Write-Host "Note: Users may need to log off/on, or restart Explorer, to see behavior changes." -ForegroundColor Yellow