While people generally talk about using macros for code generation, macros are essentially for doing things at compile time. (Or read time, in the case of read macros, e.g. 'x -> (quote x).) Due to simplicity of Lisp syntax, the easiest way to extend the language is usually by generating code, though, so that's what they're associated with.

You can do things with the C preprocessor at compile time, it's just completely unaware of C, which really cripples it. The Lisp preprocessor (its macro system) is also running Lisp, and can easily process itself.