It's not anti-aliasing, it's OS X rendering fonts as they were intended without any alteration, rather than optimising them for screen. Microsoft renders them within the boundaries of the pixel grid. Microsoft's technique looks better on small monitors, but up the size and it appears sharp and blocky. It looks weird until you get used to it, then Windows looks terrible and basic.
edit: Sounds like you're on an eMac. I can imagine it looking a bit too blurry on there. My parents use a tiny CRT monitor with their PowerMac and it drives me nuts, but my 20" iMac is fine.