Just following up to Mr. G Williams post above –
As he noted, users of Japanese (and probably Chinese and Korean versions) of Windows even in XP do not benefit from ClearType. Font rendering in XP remains for those users just as backwards as it was in Windows 3.1.
I recently remembered this problem existed when I reinstalled Windows XP the other day – most Japanese users are greeted with a UI close to the first screenshot in this page : http://www.geocities.jp/poe99/CAT/XP/page08/index.htm
For some reason I can’t remember – and a MSDN blogger even addressed this at one point, the native Japanese fonts, MS Gothic and MS Mincho, are instructed to ignore ClearType rendering at all cost.
There was a mod at one point listed that more or less required you to decompile the Microsoft-provided fonts and to recompile them without the “Ignore ClearType” flags. Once you have your freshly un-castrated files, you simply replace the files and you’re greeted with a much more pleasant reading experience found in the second screenshot of the LOVELY XP! page linked above.
However, there’s been a new development – with the release of Vista, there’s also a new typeface! A ClearType-enhanced Corbel-alike for Japanese (and possibly other versions too) users called Meiryo.
You can see the differences in the last screenshot of LOVELY XP! The author was able to get his hands on the Meiryo typeface. The implementation is not perfect due to the way Windows handles and replaces fonts on-the-fly. Windows, according to the registry uses FontLink to replace Tahoma with the appropriate font when Tahoma cannot accomodate the character set (ie displaying say, a Japanese word in the middle an English sentance)
It appears that if you simply replace MS UI Gothic with Meiryo, the typeface becomes bolded twice and muddled (last screenshot; left), versus when you simply relink Meiryo with Tahoma in the registry (last screenshot; right). I’ll look into this more when I get home. I still don’t fully understand the process or what causes the difference.