Does anyone know how to write a VxD, or where there is information on it? As I only have something small but specific in mind for the moment (I want to toggle RTS in the COM ports manually, rather than use the built-in Windows functions which are too slow), I don't want to spend an horrific amount of money on some software to help me write it (WinDriver sounded great, but they want $1200 for a licence to go on '95/'98 AND NT! - I don't think so).

What is the best (but most cost-effective) tool or software package to write a VxD with? Where can I get details of how to do it?