Problem with attachment download URL

In the attachment.module, the theme_attachments function uses the 'l' functon to generate the URL to download the file. On my installation, this results in the following sample URL:

http://localhost/drupal/index.php?q=http://localhost/drupal//files/activ...

Attempting to access this URL results in a 404, although the file exists and can be downloaded via url:

http://localhost/drupal//files/active/2/DocumentOne.pdf

Where is the problem here? Is something in my configuration causing the URL to be generated incorrectly or is this URL correct and doesn't work because of some other issue?

Thanks,

Dave