Actually I just re-downloaded IE7 without uninstalling what I thought was IE7. I did have a working IE with tabbed browsing, but when clicking on "help" > "about internet explorer" it would show I had IE6. Anyway, now "help" > "internet explorer" shows IE7, AND, tools > manage add-ons works! It appears you may have been right, IE may have been corrupted.
I probably should have uninstalled IE7? before installing IE7, but it seems to have worked..... thanks!