URL Friendly Enabled = Internal Server Error / Need Help

[xTOPSECRETx]

When I enable Use Full Friendly URLs

i get this error below when i try to click on my forum boards... how do i fix this?

Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator and inform them of the time the error occurred, and anything you might have done that may have caused the error.
More information about this error may be available in the server error log.

 

[Blandt]

The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator and inform them of the time the error occurred, and anything you might have done that may have caused the error.
More information about this error may be available in the server error log.

If you are running Apache make sure you have the rewrite mod installed and your .htaccess in place