There isn't a way for convert FTW to GEDCOM.
If the person who sent you the FTW file is still alive and you are still on good terms, the easiest way to transfer the data is to ask him/her to send you a GEDCOM, making sure to specify "PAF format".
If not, your next best bet is to find someone else who has the same version of FTM, or later, as did the person who created the FTW file. Ask them to load it to their machine, then export a GEDCOM.
20 MB is a lot. If you e-mail it to someone who has FTM, make sure they know it is coming and their e-mail won't reject it for being too big. If you have it on a ZIP drive, you might be able to find someone in your area who 1) has the proper version of FTM, 2) Also has a ZIP drive, 3) Would be willing to do you a favor. You might try a note on the bulletin board of your county library's genealogical collections room, or your LDS FHC, or in your county genealogy society's news letter. You could also try a post on GenForum's FTM board,http://genforum.genealogy.com/using/
asking if there was someone in "x" county with FTM version "y" and a Zip drive.