SQL Server Express requires .NET framework 3.5.  In some cases .NET 3.5 and/or .NET 4.0 is already installed, however, the SQL Server installer is looking for Service Pack 1 secifically.


Here is a link to .NET 3.5 SP1.  Install this version of the .NET framework and retry the SQL Server installation.

Microsoft .NET Framework 3.5 Service Pack 1