The account you are using to connect to your distributor does not have
rights to connect to it. You have to make sure that this account is in the
dbo role in the distribution database.

This may be the case if the server has been renamed. You may alternatively
see these cryptic errors :
18483 - could not connect to server 'Newname' because distributor_admin is
not defined as a remote login at the server.
18482 - could not connect to server 'Newname' because 'Oldname' is not
defined as a remote server.
Try:
Use Master
go
Select @@Servername

This should return your current server name but if it
returns NULL then try:
Use Master
go
Sp_DropServer 'OldName'
GO
Use Master
go
Sp_Addserver 'NewName', 'local'
GO
Stop and Start SQL Services
NB: You can use the @@SERVERNAME global variable or the
SERVERPROPERTY('ServerName') function in SQL Server to find the network name
of the computer running SQL Server. The ServerName property of the
SERVERPROPERTY function automatically reports the change in the network name
of the computer when you restart the computer and the SQL Server service.
The @@SERVERNAME global variable retains the original SQL Server computer
name until the SQL Server name is manually reset.

    Cheers,
         Paul Ibison SQL Server MVP, www.replicationanswers.com .