Error message: "[DBNETLIB][ConnectionOpen (Connect()).]SQL Server does not exist or access denied."

If you get "what the topic stated", you might prolly searching for the solution. I'm facing the exact problem here, however, i found the solution through dbforums.

The cause for the error persist on my computer was due to the changes of driver from sqladapter to ado.NET, to access to Microsoft SQL Server 2000. This error shows up, which, i wasted some times reviewing my code, where and what went wrong.

Some other causes of the problem are;

Actually, I've now hit this problem a few times now, usually when installing MSDE or 2000 SE on Windows 2003. The problem is around security for the slammer worm, and a dll called DBNETLIB.dll. When this file is out of date, Server 2003 and perhaps SP2 on XP (I've had a similar experience with XP SP2) will actually disable the TCP/IP protocol for SQL Server to limit an attack. The link below will take you to a fix that will examine your install of SQL Server, then apply appropriate updates. This will update the DBNETLIB.DLL which is responsible for managing the TCP/IP protocol for SQL Server. Most people will assume that installing SP3a will fix this issue, unfortunately, it does not, and I found it is ok so install this critical update after SP3a.

2) I ran into this problem after a firewall was put into place.

And the solution that i found to solves those problem;

Solution to no 1, download critical update provided by Microsoft themselves, here.

Solution to no 2, The fix that worked for me was to delete the entry for the SQL Server in the following registry location:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\MSSQLServer\ Client\ConnectTo\DSQUERY
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I chose solution no 1 and my problem fixed. My version of SQL Server was kinda old, which is the reason i choose it.

*If you have any other problem that need my help, let me know. I'm glad to help you out :)