If you are using DfontSplitter on Vista and when you convert a font, Windows then complains that it is not a valid font file, you can use this workaround to install the font into your Fonts folder. The font should then work normally.
I’ve tested this with a bunch of fonts, but it might not work absolutely everywhere. You’ll just have to try it and see if it helps.
The details of the workaround are listed here, on the dev wiki.
I’m still interested in testers for XP Service Pack 2. If you’d like to help out please get in contact via email or leave a comment below.