>what tools, what language

On Windows, get a book about VxD drivers (I don't remember the exact
title. There are two, one for NT and one for normal Windows I think).
Language is almost invariably C or C++ (using only the C subset for direct
IO related parts) with some assembly sometimes needed (inline assembly
handled by the C compiler for you normally). The book and experience are
essential imho because VxD development invariably means restarting the
machine after every compile/build and that crashes will be bad. The best
configuration is a networked machine with a bare bones Windows
installation that is used for testing only. Development is done somewhere
else.

Peter

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