A few users cannot access OWA?

We are using OWA (Exch 2k)with SSL. However, a few people cannot access their mail via OWA. I put in Domain\Username and password and get no further. If I cancel out of the Login box, I get an Access Denied error. We have checked all of the IIS/Exchange permissions and all looks good. I know it is not a browser problem because I have tried to login these problem users from my pc (and it works with my account). Has anyone had similiar problems? It would be easier to fix if everyone was having problems, but they are not. All help appreciated!