When you compile the web projects in visual studio 2005/2008, you may receive the above message.
And in order to get rid of this, you can either kill the process aspnet_wp.exe using the task manager
or add the following tag in the web.config inside system.web tag.
<hostingEnvironment shadowCopyBinAssemblies="false" />
Although the former resolution is just a temperory fix, the later one will fix this issue for that