Small tip, if you want to use SQL Server Express 2008 instead of SQL Server Express 2005 which is included in the Microsoft Dynamics NAV 2009 RTM installation, here is what to do:
The Demo installation of NAV 2009 RTM is looking for a instance called MSSQLSERVER, so if this is not detected during install, SQL Server Express 2005 will be installed, and Demo Database will be attached to SQL 2005 even though you might have had SQL 2008 Express installed.
So to avoid this set the Instance to MSSQLSERVER on the SQL Server 2008 Express. This way SQL Server 2008 Express will be used and SQL Server 2005 will not be installed.
Note: SQL Server 2008 requires the following:
.NET Framework 3.5 SP1
Windows Installer 4.5
Windows PowerShell 1.0 (Included in Windows Vista SP1 and Windows Server 2008)
Claus Lundstrøm, Program Manager, Microsoft Dynamics NAV