500 Internal Server Error

Hi

My host has just upgraded to a new server and I am now getting an "Internal Server Error" when I try to log into my CMSB admin. The rest of the site works fine though including the CMS pages - it is just the admin. - error below:-

Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator, webmaster@isltours.co.uk 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.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.
Apache/2.2.18 (Unix) mod_ssl/2.2.18 OpenSSL/0.9.8e-fips-rhel5 mod_auth_passthrough/2.1 mod_bwlimited/1.4 FrontPage/5.0.2.2635

It appears that my server no longer supports php comands in the .htaccess file. However, I am not sure this is the cause because I have renamed my .htaccess files so they are not used and I still get the problem.

Any help appreciated.

Thanks
Andy