I’d recommend using the Visual Studio 2005 Command Prompt from All Programs – Microsoft Visual Studio 2005 – Visual Studio Tools, so you don’t have to worry about setting your PATH properly.

If you do a
Devenv /?
From the command prompt, you’ll get a long usage statement.  Here’s what you need to know in order to build.

In the usage, you’ll see the following example:
devenv solutionfile.sln /build solutionconfig [ /project projectnameorfile [ /projectconfig name ] ]

The minimum you’ll need to build is
devenv <solutionname> /build

You can also specify the solution configuration and the project configuration.  You can use or create new project and solution configurations from within Visual Studio at Build – Configuration Manager.   For example:
devenv <solutionname> /build "Debug|Any CPU" /project <projectname> /projectconfig “Debug|Any CPU”

And lastly, you can specify where to write the build log to.
devenv <solutionname> /build "Debug|Any CPU" /project <projectname> /projectconfig “Debug:Any CPU” /out <filename>

Happy Visual Studio’ing!