HOWTO: Create A Shortcut To Switch Users In Windows XP/Vista/7
Of the dizzying array of keyboard shortcuts that Microsoft has for the Logo Key, none of the include a way to switch users. While Logo+L comes close (locks the screen, which allows you to also switch users) you can cut out the extra step with a simple program.
Duncan Smart created a simple C# program for those of us without Windows Business/Pro/Ultimate. Simply create a shortcut to this program and use the standard Windows way to assign a keyboard shortcut to it.