I just checked mine.
No browser sessions open.
Outlook Express open, minimized.
IE core running (28k + RAM used)
IE one instance (17K+ RAM (due to OE)
I am not sure whether Windows Messenger or MSN Messenger or something else requires an instance of IE.
As I have opened this browser window to write this, one more instance (10k+ RAM) was opened, which is what I would expect.
If you do not have an email program open, no spam washer or anything else running, no IM clients and now browser sessions, I would expect that there would be only the core of IE running. (Although some other Office Apps will open an instance of IE).
I guess I would be concerned if 3 or more instances of IE showed up in Task Manager on a clean start without ever having opened an IM, browser session, or email client.