Recently I had to make serveral XP terminals "auto-login" for guest access in common areas.  The default user had limited permissions of course.  With just a few registry keys, it was a piece of cake.  Add/Change the following keys in the "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon" hive... 

DefaultUserName - string - <enter your default username>

DefaultPassword - string - <enter the default user's pwd>

AutoAdminLogon - string - 1

Those keys when properly set will automatically log you into a given XP system with the credentials specified.  Note that you can "logout" of XP to login as another user, eg, Administrator to install applications and change system configurations.