Further to what John said about abc. There is a free program called "lillypond" that will typeset music. You write it in a language called "mudela" which is very much like abc; but a lot more powerful. The results are quite good, although it can't do guitar tab yet.

As a matter of fact it uses the TeX program that Mark mentioned to do the actual typesetting; but the mudela language is a lot more natural for musicians than the example Mark shows.

You can find out more about it at: http://www.cs.uu.nl/people/hanwen/lilypond/. It has actually been developed for UNIX; but there is a Windows32 version.

Murray