When you install SQL Server 2008 – specifically Express editions – many people have trouble working out how to get a network connection established them; they simply don’t understand why their Management Studio client rejects it saying it can not find the instance.
Well, there are two things to check:
Make sure the machine that has SQL Server (either full or Express, both have this “habit”!) has the relevant port opened.
2. SQL Server Browser
There is a little service called SQL Server Browser. If this is stopped (default for Express) then you will find that remote connections don't succeed. Start this service, and all sorted.
The only thing to bear in mind, is that by starting this service you are technically advertising the SQL Server installation on your machine; so do NOT start this service if your firewall is going to let the general public on your network connect.