Cannot install the vCenter Agent Service

Hello,
I am getting this error message saying, “Cannot install the vCenter Agent Service. Cannot upload agent”.
I got this whenever I try adding new host to my vCenter which is running ESXi 4.1.
After showing that error message my host is then disconnected. I tried reconnecting it but I get another error saying, “Cannot complete log in due to an incorrect user name or password”.
I have tried some troubleshooting here but I could make it work again.
There has been no problem when I added that new host.
Is it possible that the problem started there?
Did I forget something very basic?
I would really appreciate if you could give me suggestions.
Thanks a lot in advance.
