strange URL when correct website not found

Hi,
I appear to have a problem with some strange pornographic index type websites. When looking for a normal website which is unavailable(both IE6 and Manzilla Firefox), I would normal get a “DNS” error page. In some cases this is still the case, however i am sometimes getting a pornographic index webpage instead. Any idea what can be causing this please, and how to get rid of the porno index?
Regards,
Peter