Visual Studio 2010 isn't building before a run when there are code changes

Tools + Options, Projects and Solutions, Build and Run. "On Run, when projects are out of date" = Always build. The one below that = Do not launch.

I had this problem and it turned out to be the Configuration Manager. I had recently changed from Debug|Mixed CPU to Debug|Any CPU and the project wasn't set to build under this configuration

open your configuration manager and check out if the checkbox in the build column for your startup project is checked. I had the same problem, but the checkbox wasn't active, so i checked it and now it works!