Unless you told it to uninstall v11 during the install of v12, it shouldn't have removed it. You can have v11.1 and v12 installed side-by-side without issue. So, v11 might still be installed -- did you check in the Start --> All Programs menu?
If you did answer yes when asked to uninstall v11, simply re-download and reinstall the old version from here:
http://yabb.jriver.com/interact/index.php?topic=36448.0You don't need to uninstall v12 first, though you can if you want. The libraries are not compatible between the two versions though, so you will likely need to restore from a backup library. MC12's installer creates a backup before upgrading the library I believe so this shouldn't be an issue.
There will likely be no further updates to v11.1 now that v12 is out, unless some world-ending bug is found. So.... Unless you want to move to v12, don't use the update feature anymore.