How can I fix the fonts on my windows XP box?

Question

I'm using a corporate build of Windows XP. When I view a github wikipage, for instance https://github.com/fholm/IronJS/wiki/Compiling-from-source I get terrible display of the fonts.

If I disable the css for the font

body {
font: 13.34px helvetica,arial,freesans,clean,sans-serif;
}

then the screen is readable.

How can I fix this?

Update: Same results with both IE6 and Chrome 12