To avoid installing the sqlserver express 2008 engine that is bundled with sharepoint server 2010, you must manipulate the installation a bit. First and foremost, install sql server 2010. Then start the SP 2010 installation as part of a farm. When the installation reaches the final step, uncheck the “configure server” option. Install sharepoint 2010 sp1 (in order to drop the system stored procedure sp_dboptions dependency). Only then, can you configure the server to connect to the already installed sql server.