I try to install SQL server 2008 Express with Advanced Services. When it comes to the server configuration step, instead of getting NT AUTHORITY\LOCAL SERVICE in account name column, i have NT AUTHORITY\NetworkService. This is read only value, I can not change this value here. Why do I not see NT AUTHORITY\LOCAL SERVICE? is this happened because of sql server 2005 express already installed before? So, how can I change to "NT AUTHORITY\LOCAL SERVICE"? Do I need to set it in regedit or are there any other ways to do it?
It does not offer NT AUTHORITY\LOCAL SERVICE because that is an invalid installation account. Under LOCAL SERVICE the SQL Server won't be able to connect to AD and all sort of functionality will be broken, like anything related to EXECUTE AS and the cascade of features that use it and then the cascade of features that use features that use it etc etc.
You can go in after the install and change the service identity itself in the services console....
You can also use the Sql Server Configuration Manager after the install to make the change.