Error nº 26 - Sql server 2005 (Error locating server/instance)

 Hi,
I am trying to connect a windows 7 professional client to sql server 2005 installed on a windows 7 professional. I am getting the error nº 26 "Error locating server/instance".
Both firewalls are disable.
The ping command works in both sides and I run the portqry command on the two computers and I get a successful result .
The Browser is running in a "Network service", the protocol TCP/IP is enable with tcp dynamic ports.
My string connection is:
"Data Source=Desktop\\xxx; Initial Catalog=SII; Network Library =DBMSSOCN; User ID=admin; Password=yyy";

Where xxx is the instance name and Desktop is the name of the server machine. I am using visual studio 2010 C#.
The Remote server connections are on.  I have tried everything but the error keeps appearing.
In the server side, the program runs perfectly.  
What can I do…I am losing my mind L
Thanks