Sounds like a apache module error.
Check if Apache conf is loading the Rewriting Module.
in Apache/conf/httpd.conf you must have uncommented this line:
LoadModule rewrite_module modules/mod_rewrite.so
(in mine case about line 196)
Some questions to help you better,
- operative system you use.
- apache version.
- PHP Version, and running mode (as CGI or module)