The Windows XP Japanese fonts contain "embedded bitmaps" that implement a form of stroke reduction so that complex glyphs are readable at small sizes on screen - for this reason on XP the old fonts are special cased so that the embedded bitmaps are used.
In Windows Vista we've had time to make new fonts for Japanese, CHT, CHS and Korean designed with ClearType in mind. These were required because Avalon does not support embedded bitmaps in TrueType fonts (or old-style .FON bitmap fonts for that matter)
Further Arial Unicode has never shipped as part of Windows. It comes with Office and is rarely installed these days.
Also no there’s no current way to have different rendering paths on different displays attached to a Windows machine. We realize that this makes life difficult for some users especially those that may have a Laptop and an external LCD panel that happens to
have BGR oriented sub-pixels.
Finally I should state the obvious in that hacking our fonts violates our EULA, and care should be taken not to let such modified fonts out into the wild.
Lead PM for fonts, Microsoft Typography, Windows International