All works with with my basic Arduino until I try to connect a simple project with the HC-SR04 sensor to my computer through the USB. I have the left pin to 5 volts and the right pin to ground. The middle two pins are connected to digital pins on the Ardino. When I try to connect, the USB connection is not there. When I try to set up the circuit while attached, when I connect the 5 volts the lights go out and the USB goes dead. I tried attaching the board to a 9V battery and it got hot. Perhaps a short, but my circuit is so simple I don't see how this is possible. thanks for any thoughts...
Hi, if you look at the HC-SR04 you will see it has the pins marked, does the problem occurr when the 5V and Gnd only pins are connected.
When you say left and right pins, from which side are you looking.
If you are looking at the PCB with the two sensors looking at you, then the left is +5V and the right is Gnd.
Can you post a picture of your project so we can see how you have connected it, did you use protoboard?
Tom... 
Thank you so much for your comemnt. Your question regarding what side are you looking from prompted me to check the 5V and ground pins and they were reversed. I had spent several hours checking everything else but forgot this simple check. It works perfectly now.