permalinks

please, i need some help with setting up permalinks.
if i want to change my permalinks from
http://www.tres-adorable.com/wordpress/?p=123 (default)
into:
http://www.tres-adorable.com/wordpress/%postname%/
and i update it, my site gets an internal server error.

we have our own apache server and mod_rewrite is running.
We already tried solving the problem with changing the wp-includes/vars.php page (tip was given on the permalinks page in codex.wordpress.org) and we are kinda lost in what to do now.

the internal server error dissapears if we remove the .htaccess file.

i hope anyone can help us out with this problem
thanks a lot!