Phew! This is the first time I see this problem on Vista.
Segoe is the official UI font on Windows Vista. PixInsight will use it by default on any Windows platform if it is present on the system. Personally, I don't like it too much. It's indeed very legible, but it also has some drawbacks, especially with some character widths that make precalculation of control sizes quite a bit tricky.
I don't think this is a problem with PI, not with Segoe either, but with the font rendering algorithm on Windows. Chances are this is a problem with your video driver. Are you using the latest version?
I suppose you can disable font anti-aliasing, or select a different algorithm, in screen properties. I'll try to investigate on my part; let me know what you find.