Why don’t the fonts look the same in Windows and in Mac OS?

Font rendering is handled by the operating system’s rasterizer. While Apple’s OS X does a great job of rendering all fine details of type on screen, the Windows rasterizer is less refined, so fonts in small sizes don’t look as good as in Mac OS.

You can enhance font display on LCD screens by turning on Windows ClearType. You can further fine-tune ClearType by using the ClearType Tuner.

Follow these steps to enhance screen fonts on Windows:
How to Use ClearType to Enhance Screen Fonts in Windows XP
ClearType Tuner
ClearType Tuner PowerToy