Best practices recommend not installing Sql Server to run as SYSTEM. What is the bare minumum you need to give the user account you create for it?
Typically I create a Domain User with only the specific rights on the network which I will require the server to have (i.e. to write to the network backup drive), I then add the account to local power users or local administrators depending on what needs to be done on the machine, however this isn't required. I've installed SQL a number of times using a standard user as a Service Account but you need to ensure that the user has access to write to the resources as listed at http://support.microsoft.com/default.aspx?scid=kb;en-us;Q283811 . Its probably not as defined an answer as you wanted but I'm only a developer (not a professional DBA / System Engineer).
Mauro PS dont downmark me for saying "only a developer" :P
By default, SQL Server 2005 installation will create a security group called SQLServer2005MSSQLUser$ComputerName$MSSQLSERVER with the correct rights. You just need to create a domain user or local user and make it a member of that group.
More details are available in the SQL Server Books Online: Reviewing Windows NT Rights and Privileges Granted for SQL Server Service Accounts