When I use SQLServer 2008 Express for my development tests, I always forget which things I have to do in order to make my SQL Server 2008 Express instance available to other machines over the networks.
So to not forget that again this are the things that you have to check to be able to acces sql server express from other machines:
1. Go to the Start Menu\All Programs\Microsoft SQL Server 2008 R2\ and run SQL Server Configuration Manager
2. In the SQL Server Configuration Manager Window, expand the tree on the left and expand the SQL Server Network configuration element on the tree. Make sure that at least TCP/IP protocol is enabled.
3. Now Click on the SQL Server Services element on the tree and make sure that the SQL Server Browser service is running. It is needed in order to make other computer able to see your server.