Oh yea...forgot about the AScan not working until after your version. This should work for you to get around the ascan limitation i believe. There was also a missing '\' in one of the paths in your posted code...So make sure to check that.

Code:
cls 
$fileA = loadfile("c:\ftp\ssn.txt") 
$fileb = loadfile("c:\ftp\data.txt") 
$filec = Open(1,"c:\ftp\outdata.txt",5) 

For Each $line In $filea
   $ssn = substr($line,1,9)
   For Each $lineb In $fileb
      If InStr ($lineb,$ssn)
         ? "A SSN In file b was found In file a, writting the line To file c"
         $ = WriteLine(1,$line+@CRLF)
      Endif
   Next
Next

? "DONE - Press any key to exit"
get $

Function loadfile($file) 
DIM $fso,$f,$fs 
$fso = CreateObject("Scripting.FileSystemObject") 
$f = $fso.GetFile($file) 
If @ERROR Exit(2) EndIf 
$fs = $f.OpenAsTextStream(1) 
$loadfile = Split($fs.Read($f.size),@CRLF) 
Exit(@ERROR) 
EndFunction