I would like to create a Build Definition inside TFS 2012 Express which will simply copy all files within my project source tree to another folder on my drive. Just to underline
Add a bat file to your source folder. within the batch file add an xcopy %1*.* TargetLocation. Add an invoke process activity to your workflow, somewhere near the end. call the bat file and pass it the SourcesDirectory.
Using this post as a guide on how to start editing TFS Build Templates (or the Wrox TFS 2012 book), you need to make the following changes:
Path.Combine(BuildDetail.DropLocation, "MyOutputFolder")
Path.Combine(SourcesDirectory, "MyFileFolder")
You may need to repeat this if you don't have all your files in one folder.
I actually put something together for TFS2010 and 2013 (not 2012, unfortunately) a few weeks ago that does exactly that. Basically, anything in the workspace you define for your build just gets shoved over to the drop location.
Here's a link to the blog post where you can download them:
http://www.incyclesoftware.com/2014/06/deploying-uncompiled-resources-release-management/
For the record, I strongly recommend against using a build process template to deploy software. Don't try to overextend the build... its job is taking stuff from source control and compiling/packaging it for deployment. Use a real release management solution to actually handle deploying software.