You must use the Role Management Tool to install or configure Microsoft.NET Framework 3.5SP1

I want install SQL Express Version in Windows 2008 R2
When I Install SQL Server Express 2008, installation fail and give this error message
"need install Microsoft .NET Framework 3.5 SP1 Setup"
I Download and install Microsoft .NET Framework 3.5 SP1 Setup but fail
"You must use the Role Management Tool to install or configure Microsoft.NET Framework 3.5SP1."

From Server-> Start->Administrator tools->Server management ->Features -> Add Features->.NET Framework 3.5.1 Features

After installation, SQL intallation can contineous.