When using ASP.NET you can get this error when another application has locked a file that ASP.NET is trying to load. Rebooting the machine to clear this error is a somewhat drastic option especially if it happens frequently. The usual suspect is the indexing server when you modify the contents of the bin directory or the web.config file. This knowledge base article explains the problem and walks through resolution for the problem (by turning off indexing server, or instructing index server to ignore your ASP.NET shadow directory).
The full test of the error is something like:
Server Error in ‘/XXXXXX’ Application
Description: An error occurred during the processing of a configuration file required to service this request. Please review the specific error details below and modify your configuration file appropriately.
Parser Error Message: Access is denied: ‘xxxx’.
Brought to you y the ‘notes to self in blog’ department, but I guess it may be of help to other folks too.