[dazzlin]

I haven't tried this myself, but supposedly, there is a way, as outlined by One Tip A Day:

1. Click on the Start button and type in ‘netplwiz‘. This will open the Advanced User Accounts menu
2. In the Users tab, highlight the account you want to login to Vista automatically with, and then ‘must enter a username and password to use this computer’
3. Click on ‘Apply’. A new window will now popup asking you to enter the password of the account you’ve just highlighted. Do this, and then click ‘Ok’
4. Click ‘Ok’ on the Advanced User Accounts menu to finish

From now on, you should automatically login to the chosen account.