By default, the SQL Server service is installed to run in the security context of the server's LocalSystem account. If you will be using SQLMail, you can leave the default LocalSystem account assigned, or you can assign a user account to the SQL Server (MSSQLServer) service.
Also by default, the Services application in the Control Panel has the Allow Service To Interact With Desktop option selected for the MSSQLServer service. Do not clear this option.
Note A service running under the LocalSystem account has no user account information associated with it. This prevents certain network server software from allowing the Windows NT service to access resources. For example, Microsoft LAN Manager user-level security servers and Novell NetWare Servers will not allow a Windows NT service running under the LocalSystem account to access their file shares.