I have a few files that are used by all my LaTeX source files. I have a file of \newcommand's, which I find very convenient, because I use these \newcommand's in almost every LaTeX document I type, but I don't want to re-type them in every LaTeX document I create. So I have collected them in a single file, math_macs.tex. I put this file in a folder that MiKTeX knows to look in. Then, in every LaTeX source file, I type the line \input math_macs between \documentclass and \begin{document}, and LaTeX knows to get the contents of math_macs.tex and process them as if they were in the source code file itself. Very convenient!
Advanced users of LaTeX have all sorts of standard files that they want LaTeX to be able to find. For now, simply create, within your My Documents folder, a subfolder called, for example, Latex files. Within that, tex, then latex, then misc. Put the file math_macs.tex in the misc folder. Here is a picture of these folders:
Now tell MiKTeX to look in the folder Latex files for additional LaTeX code. Click on Start, Programs, find MiKTeX Settings. Click the tab for Roots. A folder structure like that above is called a Root directory.
Click Add and browse to select the folder which you've created to hold the .tex files. Then click OK.
