Even easier, in Firefox since version 3.0, is that you can go to Tools --> Add-Ons and there's a plugins section.
Anyway, to answer your question, I THINK IE supports the about:plugins command as well, but don't quote me on it. I'll either look brilliant if it works, or just say I thought it did but must have been mistaken if it doesn't. In IE7 you can go to the tools button, and there's a manage add-ons option. Almost positive nothing like that existed in IE6, which was a big part of the problem when it came to malware.
I have Windows XP Pro, sp3.
With Firefox you can type "about:plugins" in the location bar then hit Enter to find out what plugins are installed in Firefox. How can you find out what plugins are installed in Internet Explorer?