You haven't mentioned whether you installed a default or a named instance. See Microsoft for further details on the difference.
What you have done should work for a default instance SERVERNAME, if instead you have installed a named instance SERVERNAME\INSTANCENAME then you would also need to allow the SQL Browser Service service. Lookup the path in Windows services for this and while in there ensure that it is set to Startup Type: Automatic and it is started. The default installation path is "C:\Program Files (x86)\Microsoft SQL Server\90\Shared\sqlbrowser.exe"
If this doesn't work, ensure that TCP/IP is enabled for the SQL Server in the SQL Configuration Manager.
To summarise:
Default instance SERVERNAME:
Named instance SERVERNAME\INSTANCENAME: