Several causes.. do a search on M$ knowledgebase for this problem..

causes include .. a corrupted dll from the installation of NAV or McAfee AV's
solutions include replacing that file or Reinstalling IE6..

I don't have any links to hand.. sry.. but you are a big boy

Cheers

Edit: I will be polite and say.. if the searches are full of **** results.. I will post these links later when I am not at work..(give me about 10hrs)