Helvetica? Oh, you must mean Arial!
While setting up a font stack for a page in my site I noticed that it wasn’t choosing the fonts in the order that I had specified whenever I viewed the page in any browser on Windows (XP). For the life of me it seemed like my fonts were getting ignored by the rules. I did a little Googling and found that’s exactly what’s happening. Apparently if Helvetica is specified anywhere, but not installed, Arial is considered the best substitute regardless of what your typographic sensibilities are telling you. In fact, this registry key is to blame (Update: some machines have additional things contributing to the issue as well):
Workaround? Call up every person who will ever view your website, and politely ask them to delete this registry key. Disregard what other implications that action might cause.
I’m hoping this helps illuminate the problem for anyone who can’t figure it out.