CSS Code working on Mac browsers but not on Windows browsers?

CSS Code working on Mac browsers but not on Windows browsers?

I have the following CSS:
font-family: 'HelveticaNeue-UltraLight', 'Helvetica Neue UltraLight',
'Helvetica Neue', Arial, Helvetica, sans-serif; font-weight: 100;
letter-spacing: 1px; }
It works on all Mac browsers (Chrome, Safari) But I opened my project on
Chrome and Internet explorer on Windows, it displays the font as bold
rather than light. I'm not sure how to fix this but I need the design to
work cross platform with the design that appears on mac.
Thanks in advance.