I imagine you're setup with suPHP. In this case PHP files should have permissions of 644 and folders 755. If you do not do this then you'll receive 500 internal server errors. If things are setup that way already then it's something in your .htaccess file that is the issue.
Along with what InFloW has said, session files (sess_*) in the tmp folder can also be the cause, though this is something typically only a server admin would be able to fix. Your best bet is to check your error log and see if it's reporting anything about the problem.