This is now outdated and a proper solution now exists. Please see Using FontForge to Fix Fonts for Windows

If you have Flash, this video details the workaround - http://www.screencast.com/users/peterupfold/folders/Jing/media/b2e6c87e-a83e-4cfc-a1c2-e7358af94d23 Alternatively, read the explanation below.

There exists a bug with either DfontSplitter, (my binary of) fondu or Windows, where you can successfully convert a dfont into a bunch of TTFs, but Windows reports the TTFs as invalid. In reality, the files are fine, but Windows for some reason doesn't like them. To workaround this bug, simply right-click the generated TTF file and choose Install. In most cases, Vista should then copy the file to your Windows Fonts folder properly (after asking for confirmation) and the font will be available to Windows applications.