Start from scratch and re-create it in Word.
Also, keep the signature simple, otherwise it will break. Not all mail clients will treat it equally, so by keeping it simple, it will "break" gracefully. By simple, I mean a few lines with a name, company, title, contact information, maybe a small logo, and that's about it. You can stylize the text, but I wouldn't do any fancy tables, graphics, centering, etc. That stuff doesn't degrade gracefully too well.