Summary, give checklist:
1. Is your target server started?
2. Is your target server listening on NP? Which Pipe?
3. Has your client enabled NP? Use the same pipe to connect as Server?
4. Are you making local connection? If so, what is the instance, default or remote?
5. Did you put correct instance name in the connection string? Remember, Sqlexpress is a named instance.
6. Did you enable remote connection? Firewall? IPSec? "File and Printer Sharing" opened? Can access server?
7. Can you make basic connection by using <servername> or <servername>\<instancename>? Use sqlcmd or osql.
8. What is your repro step? What was your client APP doing during this error occuring? Which DB operation, detail?
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