How to backup SQL Server data files

I have a directory, MSSQL11.MSSQLSERVER2, that contains the data files for my local SQL Server. Even when I stop the Server so that the files are not in use, I get and error that access is deigned.

Is there a way around this? Would an upgrade to a different version of SB solve the problem?

Many thanks!! I love the product and am willing to upgrade if it would solve my problems.

Todd