The challenge:
Provide dynamic web fonts that render acceptably across all major browsers, devices and Operating Systems
The story:
I'm pretty sure the web-fonts, which look ugly, are missing some hinting (edit: Nope, even though hinting is still required to make a font look good on windows, the real problem here was the disabled ClearType in XP. However, this answer has a solution anyway).
Unlike font renderers on other operating systems the windows engine relies on hinting information shipped with the font. If the hinting is missing, bad or broken the font will look ugly - simple as that. Luckly there's a way to add some automatically generated hinting to a ttf-font using ttfautohint. After adding the hinting you can generate the different web-font-formats (eof, woff, etc) you need from the enhanced TTF. Additionally you should try to use SVG fonts if font-smoothing is disabled, since they are always rendered with antialiasing.
I've written an article on how to create web-fonts on Pixels|Bytes, which explains how to add hinting to a font and how to use my Font Smoothie script to enable SVG fonts when necessary. Hope this helps :)
PS: I'm aware that linking to my own blog won't be appreciated here, but I think my post describes a good solution to the problem, so I posted it anyway. Please leave a comment if you want me to remove the link and I'll do so.