If you are running Sql Server 2005 on Windows Server 2003 SP2, you may get this message from Sql Server native client connection:
An existing connection was forcibly closed by the remote host
This is most likely caused by a “feature” called TCP Chimney offloading. On the server, run:
Netsh int ip set chimney DISABLED
should fix your problem. It seems Visual Basic 6 (VB6) based applications and Heat Call Logging are particularly susceptible to this problem.
KB article here: http://support.microsoft.com/default.aspx/kb/945977