While I understand why you do not want to install a program to do this, would you be okay with a 3rd party command line executable which can be stored on the server where the batch file is run from, and referenced there?
(If So Try 7-Zip)
7Zip's command line files can run from the Share you call the batch file from and not cause any real network issues because the file would be executed on the local client side.
Though if you prefer not to run the file remotely, but that is not necessary for this file because it is very small.
You can DL 7Zip here:
http://www.7-zip.org/download.html(I
f Not Then See Below)
I found this VB Script which purports to do exactly what you're looking for, if it must be done without any 3rd party files of any sort, then I would try using this VB Script I have attached.
NOTE: I did not write this Script, I found it here:
http://www.visualbasicscript.com/m53086.aspx