There are a couple of problems still.

First, your SSL cert needs to be reinstalled with the correct CA bundle. That is why you are getting the trust issues when you first visit your site.

Second, that should not be popping up at all when you first visit the site, but I think the issue is that you have hardlinked an ssl link to your trustlogo image. Change that to a relative path and let osCMax determine whether or not to call it via http or https.

If you fix the certificate, that will correct the error though.

Next, your site is still having problems swtiching from SSL to non SSL in your categories. I am betting this is because the site is not using the standard categories box, but uses its own linking instead. In that case the links are not dynamically generated and will always have issues with switching. In this case you should hardcode the links to use http:// in the url...

You may want to see this thread too:
The SSL In OsCommerce Guide For The Innocent - osCommerce Community Support Forums