I've used freePCB before, and its pretty good for a windows based PCB layout solution.

On the other end of the free spectrum is gEDA. gEDA is a complete Electronic Design Automation suite, which covers tasks from circuit design and simulation all the way through to board layout. Its free (GPL), and a great option for POSIX platforms (Mac OSX, Linux, Unix, etc.).

I've heard that gEDA's layout software, PCB, will run on Windows, but I haven't tried it. It will need to be compiled from source, and run with an X server.