Drupal on a virtual subdirectory - forms not posting correctly

Hi folks,

I'm using ISAPI Rewrite 3 on an Acquia Drupal site, on IIS6.

The external url of the site is something like this:

http://site.domain.com/directory

The site itself isn't actually in a subdirectory, but the url has to have that structure to conform with our firewall/web access control system.

By setting the $base_url parameter in Drupal's settings.php file to the url above, I have the site mostly working fine.

One issue is that forms on the site (like the search form, the login form, etc..) post to http://site.domain.com instead of to http://site.domain.com/subdirectory.

I've tried to set RewriteBase in .htaccess to /subdirectory, but the effect of that is that all links on my site return a 404 error.

Any ideas?

Thanks in advance.