One other thing -- do you have a previous version of Helvetica on your system? If so, it could be conflicting with the new LT one. Having duplicate fonts, even ones differing in format (TT vs. PS vs OT) has been the culprit in other font-related problems. Sometimes the issue seems to be related to the font's internal name(s) too, so it's not just an issue of the Windows-visible font filename, there are additional internal names that are variously important.
Had you previously used any Helvetica fonts at all, especially one that was a printer-resident version of Helvetica? I've experienced situations where if a printer-resident font was used in the doc but then it was saved and eventually opened when a different printer was set as the default, and even though I had the "real" font installed on my system, the font display would get clobbered. I always ended up removing the font from my system, rebooting, then reinstalling the font and making sure that Distiller knows the location of the font.