How to use Popen in Windows to invoke an external .py script and wait for its completion

#**********************************************************************
# Description:
#    Zips the contents of a folder.
# Parameters:
#   0 - Input folder.
#   1 - Output zip file. It is assumed that the user added the .zip
#       extension.
#**********************************************************************

# Import modules and create the geoprocessor
#
import sys, zipfile, arcgisscripting, os, traceback
gp = arcgisscripting.create()

# Function for zipping files.  If keep is true, the folder, along with
#  all its contents, will be written to the zip file.  If false, only
#  the contents of the input folder will be written to the zip file -
#  the input folder name will not appear in the zip file.
#
def zipws(path, zip, keep):
    path = os.path.normpath(path)
    # os.walk visits every subdirectory, returning a 3-tuple
    #  of directory name, subdirectories in it, and filenames
    #  in it.
    #
    for (dirpath, dirnames, filenames) in os.walk(path):
        # Iterate over every filename
        #
        for file in filenames:
            # Ignore .lock files
            #
            if not file.endswith('.lock'):
                gp.AddMessage("Adding %s..." % os.path.join(path, dirpath, file))
                try:
                    if keep:
                        zip.write(os.path.join(dirpath, file),
                        os.path.join(os.path.basename(path),
                        os.path.join(dirpath, file)[len(path)+len(os.sep):]))
                    else:
                        zip.write(os.path.join(dirpath, file),
                        os.path.join(dirpath[len(path):], file))

                except Exception, e:
                    gp.AddWarning("    Error adding %s: %s" % (file, e))

    return None

if __name__ == '__main__':
    try:
        # Get the tool parameter values
        #
        infolder = gp.GetParameterAsText(0)
        outfile = gp.GetParameterAsText(1)

        # Create the zip file for writing compressed data. In some rare
        #  instances, the ZIP_DEFLATED constant may be unavailable and
        #  the ZIP_STORED constant is used instead.  When ZIP_STORED is
        #  used, the zip file does not contain compressed data, resulting
        #  in large zip files.
        #
        try:
                zip = zipfile.ZipFile(outfile, 'w', zipfile.ZIP_DEFLATED)
                zipws(infolder, zip, True)
                zip.close()
        except RuntimeError:
                # Delete zip file if exists
                #
                if os.path.exists(outfile):
                        os.unlink(outfile)
                zip = zipfile.ZipFile(outfile, 'w', zipfile.ZIP_STORED)
                zipws(infolder, zip, True)
                zip.close()
                gp.AddWarning("    Unable to compress zip file contents.")

        gp.AddMessage("Zip file created successfully")

    except:
        # Return any python specific errors as well as any errors from the geoprocessor
        #
        tb = sys.exc_info()[2]
        tbinfo = traceback.format_tb(tb)[0]
        pymsg = "PYTHON ERRORS:nTraceback Info:n" + tbinfo +
                "nError Info:n    " +                 str(sys.exc_type) +
                ": " + str(sys.exc_value) + "n"
        gp.AddError(pymsg)

        msgs = "GP ERRORS:n" + gp.GetMessages(2) + "n"
        gp.AddError(msgs)