Webfonts not matching between OS X and Windows

My personal website uses Helvetica, which is defined as the font family in my CSS. The site looks fine on OS X and iOS on all browsers, but when I view it on my Windows 7 machine it doesn't seem to render the text the same and basically looks...bad.

As I have very little knowledge about web fonts I was hoping someone could explain why different operating systems render fonts differently on the web, and is there any code tricks to present a unified style on all platforms.

Thanks for any help.

Here's the site by the way: colm.io