If you're logging into a computer that's on a domain and you go to "Switch User", it will try to log you in as a domain user. So, if you want to log in as a local user, you have to specify that.
However, there's no "Log on to" drop-down box anymore, so you have to type the computer name followed by a backslash in front of the username.
Okay, but what if you don't know the computer name?
Aha! There's a shortcut. Instead of the computer name, use a period (.). That's right, a dot. A dot is shorthand for "where I am now" in UNC paths, DOS and UNIX/Linux. So, you type the username like this: