I don't know how to install them once you've downloaded them ...

I don't know about IE7, but with Firefox the plugins' extensions need to be changed from .xml to .src. Then you can use window.sidebar.addSearchEngine, which is the subject of an article at mozilla.org called Using JavaScript to add new Search Engines.

There are already several OpenSearch plugins for drupal.org at the Mycroft Project. You can look at their JavaScript installer to see how it works.