But I haven't read David Eddings for ages.

This is unusual. Given that your Belgarion account is an Admin capable account, then the inbuilt System Administrator account should have disappeared from view.

How do you log into the Belgarion account if it is not displayed in the Welcome Screen? Has it ever been available? Does XP boot up directly into the System Administrator account, or do you have to accept or input a password?

I would password protect the Belgarion account anyway, and just double check that it is an Admin capable account.

Have a look at TweakUI for XP from http://www.microsoft.com/windowsxp/downloads/powertoys/xppowertoys.mspx

It has some useful account and log-on options.