Disable the login prompt for password in XP

1. Click Start and Run
2. Type "Control Userpasswords2" and press enter.
3. Uncheck the box for "Users must enter a user name and password to use this computer."
4. Click Apply and Ok.

Refer:http://support.microsoft.com/kb/315231/en-us

Disable prompt for password when XP resumes from standby or hibernation

1. Click Start, Settings, and Control Panel
2. Double-click Power Options
3. Uncheck the "Prompt for password when computer resumes from standby" option.
4. Click Apply and then Ok.
   
   Disabling the Windows XP Welcome screen

1. Click Start, Settings, and Control Panel.
2. Open User Accounts.
3. Click "Change the way users log on or off".
4. Uncheck the "Use the Welcome screen".
5. Click "Apply Options"