$yourdir="c:\temp\"
$destfile= ([Environment]::GetFolderPath("Desktop") + "\totalresult.txt")
Get-ChildItem $yourdir -File -Filter *.txt | %{"________________________" |out-file  $destfile -Append; $_.Name  | Out-File  $destfile -Append; gc $_.FullName | Out-File  $destfile -Append}