Rendering of web fonts on Windows XP is one big issue that there are solutions for but it really takes a lot of tweaking. First, it starts with a well designed and created font. Second, hinting can be applied that helps shift around the pixels on the screen to better map the outlines to the pixels on the screen, ClearType does help in some instances but mostly with small text, using tricks like sending WOFF/CFF to turn on GDI grayscale anti-aliasing is another that is employed when a font is a certain size - usually larger sizes.
Its one big issue and one that all of us web font service providers are trying to help solve. If we all had the resources we would be sending down one specific version for each browser / OS and screen setting combination. But that is pretty prohibitive. We are starting to see better renders built into the browsers and some tweaks that the folks at Webkit, Mozilla and IE are making.