No connection could be made because the target machine actively refused it.

I faced this situation yesterday .
We were using Tivoli to connect to SQL Server 2005 (Clustered instance) to take backups.

But it was failing with the error (not printing the complee error here):
No connection could be made because the target machine actively refused it.

I asked myself why the server is refusing the client request ??
Since the connection was failing , this issue had to do with connectivity .

I started checking few things :

1) Port on which SQL is listening
2) named or default instance
3) Protocols enabled @ server level and @ client level .

It was a default instance on some port other than 1433 (as hackers can catch it easily).We found that it was SQL Server that was using this port ...how ??
By connecting to SQL Server through management studio using

tcp:servername\instancename , portname

and it connected .We also did netstat -aon and or -aonb(this takes a lot of time as it also finds the exe name) and confirmed that only SQL Server is using this port.

Then we checked SQL Server Client network utility (cliconfg) and found that no protocols were enabled .We enabled named pipes and TCP/IP (not sure how it got disabled).

Tried to connect tivoli which failed again .Finally we forcibly made tivoli to connect to SQL Server usin gthat port by creating a TCP/IP alias ...
This worked and our issue got resolved ..

In past I found that the port SQL Server is using is either blocked or being used by other application .I that case stop that application and create the alias .

Hope this helps !!!

Regards
Abhay