This just looks like the normal ugly way fonts are rendered in WinXP. Some (IMO: misguided) people even prefer it.
To get anti-aliasing for desktop fonts in general on XP you have to turn it on, from Display Properties -> Appearance -> Effects -> Use the following method to smooth edges of screen fonts -> ClearType. The default setting “Standard” is the old-school Windows “font smudging” technique that only bothers to turn on at larger font sizes, and then often makes a mess.
IE7+ has an option—on by default—to always use ClearType anti-aliasing to render fonts in the web browser. Other web browsers will respect the user's configured font rendering method. It is a shame that so many people still have this beneficial setting turned off, but it's not really your problem.
(There is nasty hack to make Chrome perform some anti-aliasing on text, which is:
text-shadow: 0px 0px 1px rgba(0,0,0,0);
but I seriously wouldn't recommend it.)
One thing you can do when the “Use the following method...” setting is set to “Standard”, to try to make the font get some form of anti-aliasing, is to check that the font in question doesn't have a GASP
table telling old-fashioned TrueType renderers to disable anti-aliasing at particular font sizes. You can change the GASP table using a font editor or with the ttfgasp.exe
command-line tool.