Installing SQL server 2005 Domain account or the Built in system account??

I am installing SQL server 2005 standard edition. About 3 installation dialog boxes into the install I get prompted with a dialog box to make several choices. The choices are confusing and I don't know which one to pick. First of all there is a check box labeled "Customize for each service account".
Then there is a drop down listbox with options of "SQL Server","SQL Server Agent", and "Analysis Services".
Also with radio buttons I am prompted to chose the following choices:
(Radio Button) Use the built in system account (Choose Network service or Local System)
Or I can use a domain user account and I am prompted to enter the Username Password and Domain.(What do they mean by domain)

Ok, so why am I getting prompted for all of this information? Is it because I chose to install Analysis services? If so, maybe I shouldn't install it?

Solution: Installing SQL server 2005 Domain account or the Built in system account??

The installation asked these parameters for account on which these service will be executed.
If you are not sure about what can be given, Just Use Buiilt in System account.
but. What my opinion is to run sqlservices using a separate domain account (3rd option).
Please Refer:
http://www.sqlserverclub.com/essentialguides/how-to-install-sql-server-quick-guide.aspx