fyn1 Please follow the troubleshooting procedure outlined here and provide us with more information. This part is especially important:
Copy any messages to a text file and jot down a few notes about when the error occurred, what you were doing at the time, and so on. Be sure to include any insights you may have gleaned about the conditions under which the issue does and doesn’t occur. Add as much information as possible about your server environment: OS version, web server version, PHP version and handler, et cetera.
If your site is accessible via the internet, a URL would be useful as well.
EDIT: Just on a hunch ... Have you also followed the instructions under "URL Rewriting" in the installation guide?