Are the PC's on a domain? Is it in a school?
If it not on a domain then you could just remove the passwords from the machines and just have the one admin account so that they can login to the default account without requesting a password. Obviously there are massive security concerns about doing it this way and it is not advisable
Another option is to use TweakUI to configure automatic logins - Again its not advisable if the machines have any delicate information on them.
See: Configure Windows XP to Automatically Login