In general, I agree that Helvetica looks much prettier on a Mac than on Windows (one of the main reasons I despise seeing font-family:Helvetica,Arial,sans-serif; on pages). Some other fonts don't look too bad on Windows, though.

I don't know much about font rendering; this is just what I've noticed. For what it's worth, some screenshots of the same text with font-family set to Helvetica [0], Myriad Pro, and Segoe UI on Windows 7, Firefox.

Helvetica(/Arial): http://snag.gy/Jneor.jpg Myriad Pro: http://snag.gy/f2IJx.jpg Segoe UI: http://snag.gy/ujvpX.jpg

[0] This actually defaults to Arial, I believe, because afaik Helvetica is not bundled with Windows. (Is there no way to use asterisks on HN?)