SQL Server does not exist or access a bug in Windows denied

SQL Server often appears with the following error:
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SQL Server Enterprise Manager
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SQL Server registration failed due to a connection error as shown below. Do you still want to register?

SQL Server does not exist or access is denied
ConnectionOpen (Connect ())
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The situation is as follows:
A machine for SQL Server SP4 system login account: admin password: A
b Machine for SQL Server 2000 system login account number: admin Password: A
c Machine for SQL Server SP4 when B machine access to a machine system login account: admin password: C
Found that a can access B of the SQL Server,c not, will be a machine's system password for D, also not. Make B a SP4 and fix it.
The summary bug is as follows: When two machine systems log on to the same account and password, this error does not occur (unless some network reason).