#NB: will cause an infinite loop

$f = 'E:\fsw' #original folder to monitor
$file1 = 'fws.txt' # filter to monitor with.
$file2 = 'C:\file.txt' #file to copy over

$fsw = New-Object IO.FileSystemWatcher $f, $file1 -Property @{NotifyFilter = [IO.NotifyFilters]'FileName, LastWrite'}
Register-ObjectEvent $fsw Changed -SourceIdentifier FileChanged -Action {
  $name = $Event.SourceEventArgs.Name
  $changeType = $Event.SourceEventArgs.ChangeType
  $timeStamp = $Event.TimeGenerated
  Copy-Item $file2 -destination $f\$file1 -force
}

# launch the following to remove the event:
# Unregister-Event FileChanged