I'm running XP pro sp3 with IE8 latest updates, and spmenu.js is loaded. But, it does generate errors on localhost that it doesn't in live production site.
I'm converting all my skins' menus to DNN Nav to avoid issues with Solpart since it is now "retired." You would need to upgrade your site, though, through one or even better two major version upgrades and I'm not sure the best approach for that. But, just for the sake of security fixes, it's definitely time to upgrade that version 3 to something more current, not to mention browser compatibility issues an upgrade should resolve. At this point, it would be a lot of major time consuming work for you to do so, though, and the skin you are using may not survive an upgrade to a more current release... but maybe a new "makeover" for the site at the time of upgrading would be a good thing, too, can't fight progress forever, right?
Anyway, I get the distinct feeling that Solpart just shouldn't be used anymore at all, not even if your site's skin relies on it. There will be more and more issues with it as time goes on, and I imagine at some point it won't be supported at all in DNN, not even for "grandfathered-in" skins.