Thanks for your post, I had the exact same problem
First I got the nsdhelp.dll error and later on the http error so I did a full install of 9.0.1, Really strange that the full installer did not ask to repair. It just did a full install as if there was no Domino installed.
anyway, direct after the full install I run the 9.0.1 -> 9.0.1FP3 installer and now I am up and running again.
As I did a full install the java.policy file was overridden so I needed to grant access to some of my xpages and agents.