A PowerShell script to list external services that have been installed separately from the base Windows experience. I.e., user-installed services.

List-External-Services.ps1

foreach ($service in Get-Service | ForEach-Object { $_.Name }) {
  $path = Get-ItemProperty -Path Registry::HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\$service | ForEach-Object { $_.ImagePath }

  # Change this as you see fit, but it should catch most externally installed services.
  if ($path -like "*C:\Windows*") {
    continue
  }

  "{0}: {1}" -f $service, $path | Write-Output
}