The following is a way to open a folder window on the desktop in both Mac and Windows:

on mouseUp
  answer folder "Get a folder:"
  if it <> "" then
    OpenFolder it
  end if
end mouseUp

on OpenFolder pPath
  switch (the platform)
  case "Win32"
    set the hideConsoleWindows to true
    if char -7 to -1 of the shellCommand is "cmd.exe" then
      replace "/" with "\" in pPath
      get shell("start explorer.exe /root," & quote & pPath & quote)
    else
      get shell("start" && quote & pPath & quote)
    end if
    break
  case "MacOS"
    if the systemVersion >= 10 then
      get shell("open " & quote & pPath & quote)
	else
      put "tell application " & quote & "Finder" & quote & cr & \
        "activate" & cr & \
        "open folder " & quote & ConvertPath(pPath) & quote & cr & \
        "end tell" into tScript
      do tScript as AppleScript
    end if
    break
  end switch
end OpenFolder

function ConvertPath pPath
  if char 1 to 9 of pPath = "/Volumes/" then
    delete char 1 to 9 of pPath
  else
    put line 1 of the drives before pPath
  end if
  replace "/" with ":" in pPath
  return pPath
end ConvertPath

Enjoy!

Posted 10/23/2002 by Ken Ray
Revised 10/24/2002 by Ken Ray
Revised 3/26/2007 by Ken Ray (fixed bug where paths with spaces didn't work on OS X; thanks to Scott Rossi for finding this one)
Revised 3/26/2007 by Ken Ray (replaced old "shortcut to folder" approach with "explorer.exe" approach - thanks to Scott Rossi for finding this one as well... :-) Revised 4/2/2007 by Ken Ray (fixed code for matching 'cmd.exe' - thanks to Scott Rossi again! :-)