You are looking for
netplwiz. You can do it irrespective of the number of accounts on the PC. You'll have to choose the one that logs in automatically of course.
This is a good how-to;
http://www.howtogeek.com/howto/windows-vista/make-windows-vista-log-on-automatically/
PS: And it can have a password, you just wont be prompted for it at login. Still better to set one for security reasons. An account with no password would be easier to get into by other means.
Also, I'd recommend the account that logs in automatically is a standard user and not an administrator.