Cannot connect from computer to server database

I'm trying to connect from my computer to a server which has a SQL Server 2008 instance installed. My computer and the server are in the same domain and the name of the server is "SERVER". The instance name is "SZSQL". If I try to connect on the server in SQL Management Studio 2008 I'm able to login using instance SERVER\SZSQL, user sa and the admin password. When I try using these login data in MS Visual Studio 2012 Server-Explorer, I get an error (translation below):

In English:

A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified).

Like I told you before, the instance name, username and password are correct when I try to login on the server itself. Just the connection from my computer to the server is not working.

Any suggestions? :)

EDIT: The Server is running on Windows Server 2008 Standard