I have installed new site in Joomla 1.7 and its working fine, but when i checked its back-end (administrator) got 500 Internal Server Error.
Note: Site
Check your error logs and backtrace what is causing the error. It sounds like you have MySQL errors, make sure any of your components/modules/aren't tricking out your site.
Think it's a component?
You can login into PHPMyAdmin and disable/enable modules manually by unpublishing them using the table editor, changing isPublished to 0 .