Mailto link spanws multiple IE sessions

When following a mailto hyperlink from any Office XP
application, instead of opening a new message in Outlook
(default Email client) it spawns about 40 Internet
Explorer sessions, most of them with an empty URL and a
few with the mailto link in the address bar.
Any ideas?
I'm in Windows 2000-SP2, Office XP-SP2, IE 6.0-SP1 (other
boxes are in IE 5.5-SP2)
Thanks for your help!