Trouble Setting Up a Simple Circuit

I have a circuit setup that is similar to this:

(“button” tutorial from the Arduino website)

but I don't use a pull-up resistor? Also, rather than lighting a LED as output, I send a message through the USB serial port.

When I close the switch, the Arduino board will power down. I attempted to fix this by connecting a 9v external power supply. The board does not power down, but now I lose my USB connection to my computer. Is this expected behavior? Does it have something to do with the fact that I don't have a pull-up resistor connected? I thought the resister was only there to ensure that voltage on the input was either 5v or 0v? (Forgive me if this is a stupid question, I don't have an electronics background).

If you are not using a resistor, but are connecting the top of the switch to 5v then you will be shorting the 5v to GND when the switch is closed. This is why the board acts funny.