IE Homepage piracy

I have tried 3 times to eliminate all traces of this rogue software and it keeps coming back and changing the Homepage in IE 6.0. We assume it is a porn site, due to the implied nature of the content. We don’t care what it is, we just want to get rid of it! It keeps changing to:

C:\Program Files\Startportal\Portal\portal.html.

My staff & I have wiped out what looks like all the entries in the registry and deleted the folder, “Startportal.” Then emptied the recycle bin. We turn off the system, then boot up. Launch IE and it’s gone for a couple of hours, Some time later, we launch IE again, and it’s back. Any clue as to what mysterious program/script this may be. We assume it came in through our email and an attachment was opened.