Internet Explorer not opening webpage

My boss is the only one who can’t access the login screen of our Internet Banking transactional website. He’s using Win NT 4.0 sp6a. He gets a page cannot be displayed error with a filename DNSERROR.HTML IN THE URL FIELD. I tried upgrading to IE 6.0 from 5.0 and still he can’t access the login screen of our website. How do I totally remove Internet Explorer 6.0 and does anybody think that there might be another problem?
Thanks.