在Ubuntu Server 12.04上设置Web应用程序会导致内部服务器错误

I have an existing web application (currently working on Windows 7). I set up a box with Ubuntu Server 12.04 that I wish to host/test my application on. Problem is, when I try to access the app I get an 500 Internal Server Error.

To be sure there is something wrong with the app itself (as opposed to Ubuntu and Apache configurations) I set up a test domain on the Ubuntu Server which I can access from my windows computer without a problem. I then ftp'd the app to this folder.

This app is using Code Igniter. I have changed the config file to reflect any changes that may affect names/paths. I have a database that is set up correctly. Have I forgotten to change anything else that would cause this error when transferring from Windows to Ubuntu?

edit: As suggested by Steve V here is a message I get in my error log when trying to access the site.

[alert] [client xxx.xxx.x.x] THEPATH/.htaccess: Invalid command 'RewriteEngine', perhaps misspelled or defined by a module not included in the server configuration

Your rewrite engine isn't enabled. Go to shell and do the following:

a2enmod rewrite
service apache2 restart