this is a generic windows issue. usually noticed by users who switch over to it after using a mac for a while. lets just say font-aliasing is not one of windows strengths.
Whilst you're right that Windows fonts are generally rendered poorly in comparison to Mac/Linux, this is especially an issue with web-fonts afaict. Web-safe fonts like Arial, Georgia etc look (pretty much) fine under Windows.
In fact I think something else is at work here, because actually the main font this site is using is 'Lato' which looks just fine under Windows if you search for it at http://www.google.com/webfonts
In fact I think something else is at work here, because actually the main font this site is using is 'Lato' which looks just fine under Windows if you search for it at http://www.google.com/webfonts