Read this for background
In short, Apple and Microsoft use very different methods to render fonts on Mac OS X and Windows. Both systems have their merits, and frankly a lot of the time people prefer the one they are used to, and when the see the other they dislike it not so much because it's bad, but because it's different.
Ordinarily this isn't an issue, Mac users have their way, and Windows users have their way. However, for Safari on Windows, Apple decided in their wisdom to haul over the Mac OS X font rendering mechanism, which is why pages loaded in Safari look not only different to the same page rendered in other web browsers, but different to every other Windows app you have.
Personally, while (likely for the reasons I stated earlier) I prefer the Mac way of doing things, I think that they should not attempt to enforce this on Windows users who are likely to see it as different first, and dislike it. Even if they like it, it's inconsistent. Even though I prefer it, I don't want to see it on Windows, as I prefer consistency more.
So, to answer your question, what's happening is that Safari is attempting to render this font in a completely different way to the other browsers, and occasionally it comes across a font or typeface that it frankly handles badly compared to Windows own technology (the opposite happens too, it's horses for courses), this can happen if a font is specified that has been tweaked to work particularly well with one system or another.