do exactly the following:
first when you boot up,enter your regular username at the logon prompt,if
you don't have a password,leave it clear and click ok,not cancel,if you do
have a password,then enter it and click the ok button.
open the control panel (startmenu>settings>control panel)
in the control panel,double click to open users,delete all users that exist
besides the one you've logged with(so there will only be one user for the
machine)
then open passwords:
if you do have a password for the existing user you used to log in with,then
click on change windows password button,enter the old password,and for the
new password,leave it blank and click ok,if you already don't have a
password then proceed to the next step.
click on the users profiles tab then select all users of this computer use
the same preferences etc,click the ok button.
restart,you shouldn't be prompted for a logon password again next time you
boot.

good luck.