Please take backup & do at your risk.
Please follow these steps to automaticaly login to XP / Vista system on boot.
Click -> Start -> Run
Type control userpasswords2
Deselect checkbox "Users must enter a user name and password to use this computer."
Click OK
Enter the username by which you want to login automatically.
Enter the password & confirm it.
Click OK.
You need to restart the system for changes to take effect.