Create a Dedicated Local Admin Account for Windows LAPS to Manage

<#
.SYNOPSIS
    Creates (or repairs) a dedicated local admin account with a random, never-seen password, ready for Windows LAPS to manage.
.DESCRIPTION
    Makes sure a named local account exists, is enabled, and is in the local Administrators group.
    When it creates the account, it gives it a long random password generated on the device and never
    written, logged, or returned. Nobody knows it, and nobody needs to: point your Windows LAPS policy's
    AdministratorAccountName at this account and LAPS takes over the password from there.
    Optionally disables the built-in Administrator (RID 500). Safe to run again; it only fixes what's off.
.PARAMETER Name
    The account name. 20 characters max, no \ / [ ] : ; | = , + * ? < > @ or quotes.
.PARAMETER Description
    Description shown on the account.
.PARAMETER DisableBuiltInAdministrator
    Also disable the built-in Administrator account.
.EXAMPLE
    .\Initialize-LapsAdminAccount.ps1 -Name lapsadmin -WhatIf
.EXAMPLE
    .\Initialize-LapsAdminAccount.ps1 -Name lapsadmin -DisableBuiltInAdministrator
#>
[CmdletBinding(SupportsShouldProcess)]
param(
    [Parameter(Mandatory)]
    [ValidateLength(1, 20)]
    [ValidatePattern('^[^\\/\[\]:;|=,+*?<>@"]+$')]
    [string]$Name,
    [ValidateLength(0, 48)][string]$Description = 'Local admin managed by Windows LAPS',
    [switch]$DisableBuiltInAdministrator
)

function New-RandomSecureString {
    param([int]$Length = 32)
    $chars = [char[]]'ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz23456789!#%+-=?@_'
    $bytes = New-Object byte[] $Length
    $rng = [System.Security.Cryptography.RandomNumberGenerator]::Create()
    $rng.GetBytes($bytes); $rng.Dispose()
    $secure = New-Object System.Security.SecureString
    foreach ($b in $bytes) { $secure.AppendChar($chars[$b % $chars.Length]) }
    $secure.MakeReadOnly()
    $secure
}

function Write-Step {
    param([string]$Step, [string]$Status, [string]$Detail = '')
    [pscustomobject]@{ ComputerName = $env:COMPUTERNAME; Account = $Name; Step = $Step; Status = $Status; Detail = $Detail }
    if ($Status -eq 'Failed') { $script:failed = $true }
}

$failed = $false
$adminsGroup = Get-LocalGroup -SID 'S-1-5-32-544'   # "Administrators" in any language

# 1. The account itself.
$user = Get-LocalUser -Name $Name -ErrorAction SilentlyContinue
if (-not $user) {
    if ($PSCmdlet.ShouldProcess($Name, 'Create local account with a random password')) {
        try {
            $user = New-LocalUser -Name $Name -Password (New-RandomSecureString) -Description $Description -PasswordNeverExpires -AccountNeverExpires -ErrorAction Stop
            Write-Step -Step 'Create account' -Status 'Done'
        }
        catch { Write-Step -Step 'Create account' -Status 'Failed' -Detail $_.Exception.Message }
    }
    else {
        Write-Step -Step 'Create account' -Status 'WhatIf'
        Write-Step -Step 'Add to Administrators' -Status 'WhatIf'
    }
}
else {
    Write-Step -Step 'Create account' -Status 'AlreadyExists'
    if (-not $user.Enabled -and $PSCmdlet.ShouldProcess($Name, 'Enable account')) {
        try { Enable-LocalUser -Name $Name -ErrorAction Stop; Write-Step -Step 'Enable account' -Status 'Done' }
        catch { Write-Step -Step 'Enable account' -Status 'Failed' -Detail $_.Exception.Message }
    }
}

# 2. Administrators membership. Add-LocalGroupMember tells us if it's already there, which dodges
#    Get-LocalGroupMember's habit of failing on groups that contain orphaned SIDs.
if ($user -and $PSCmdlet.ShouldProcess($adminsGroup.Name, "Add $Name")) {
    try {
        Add-LocalGroupMember -Group $adminsGroup -Member $user -ErrorAction Stop
        Write-Step -Step 'Add to Administrators' -Status 'Done'
    }
    catch [Microsoft.PowerShell.Commands.MemberExistsException] { Write-Step -Step 'Add to Administrators' -Status 'AlreadyMember' }
    catch { Write-Step -Step 'Add to Administrators' -Status 'Failed' -Detail $_.Exception.Message }
}

# 3. Optionally switch off the built-in Administrator, but never the account we just set up.
if ($DisableBuiltInAdministrator) {
    $builtIn = Get-LocalUser | Where-Object { $_.SID.Value -match '^S-1-5-21-.+-500$' }
    if ($builtIn.Name -eq $Name) {
        Write-Step -Step 'Disable built-in Administrator' -Status 'Skipped' -Detail 'That is the account you named.'
    }
    elseif (-not $builtIn.Enabled) {
        Write-Step -Step 'Disable built-in Administrator' -Status 'AlreadyDisabled'
    }
    elseif ($PSCmdlet.ShouldProcess($builtIn.Name, 'Disable built-in Administrator')) {
        try { Disable-LocalUser -SID $builtIn.SID -ErrorAction Stop; Write-Step -Step 'Disable built-in Administrator' -Status 'Done' }
        catch { Write-Step -Step 'Disable built-in Administrator' -Status 'Failed' -Detail $_.Exception.Message }
    }
}

exit ([int]$failed)