Outlook Express is an integral part of (pre-Vista) Windows. It doesn't exist as a program in its own right, so there's no way that you can acquire it from anywhere.
If the computer without Outlook Express is running Windows Vista, you should use Windows Mail instead. (alternatively, use Windows Live Mail or Thunderbird. See below).
If the computer without Outlook Express is running Windows XP (but has somehow 'lost' Outlook Express or is one of the fairly rare computers which never had it in the first place), you can use Windows Live Mail. (That's not the same as Windows Mail). It's a direct replacement for Outlook Express and will run under XP:
http://get.live.com/wlmail/overview
That, of course, assumes that you want a Microsoft product. Thunderbird is free and, in many people's opinions, far better than any Microsoft email client:
http://www.mozilla.com/thunderbird/
Chris