These methods should work for XP, Vista and Windows 7

1. Go to the Start Menu and the Run box.

2. Type in the following and click OK:

control userpasswords2

3. In the new Windows that appears select the account you wish to make the primary logon.

4. Now uncheck the "Users must enter a username and password..." box.

5. Hit Apply and a dialog box will appear asking you to confirm the selected users password.

6. Click OK and you are finished.

For Vista, if the above does not work try:

a) click Start and type ‘netplwiz’ or click that name on the Start menu
b) in the GUI window that comes up, uncheck the “Users must enter a username and password to use this computer” box
c) click ‘Apply’
d) in the new dialog box that opens type the name of the account you want to auto-logon by default (if the account is not normally passworded then it blank)
e) click OK and exit