rewrite shorter url

[masterkain]

hi there,
I need to point to webserver documentroot an url, for example:

http://www.foo.bar/modules/defaultmodule/

to

http://www.foo.bar/

in this case in

http://www.foo.bar

exist an index.php that redirects to the default module (I'm using xoops cms), so every time a user digits

http://www.foo.bar

it results in a

http://www.foo.bar/modules/defaultmodule/

i think there are free ways to do this:
frames
apache directives
php

I'm just trying apache directives with mod_rewrite but I've no luck with it.
can you please give me some hint to do this?
TIA