If you prefer the fast way (less secure), Vista does provide an easy GUI for configure this.

Vista: control userpassword2

Here is the what you have to call. I just wonder why they don't provide this in normal User Account page. Then you just to select the account you want to access automatically, then uncheck this:-

User Accounts

It will ask you a password of the selected account. Then next time, you restart the computer, you will never have to put a password anymore :-D

Update before Win 7 launch (Oct 19, 2009): Although this command works with Vista and XP, it doesn't seem to work with Windows 7. The alternative is using 'netplwiz' instead of 'control userpasswords2' and yes,  netplwiz works on Vista and XP too. Quite good, huh?