Wots yer OS?
Since NT, and therefore in Windows 2000 and Windows XP it has been possible to set up "AutoAdminLogon" if there is to be a default user. Since they beefed up security in XP it has been necessary to do a Registry edit.
Do a Google on AutoAdminLogon. That'll get you the link to the Microsoft Knowledge Base document to tell you how to do the business.
If no luck, I'll find it for you. fobs