communicate between a desktop XP and a arduino uno

Using a usb connection from my desktop XP compter to my arduino uno, can I communicate back and forth. For example, using a progaram in C# in my computer, I would like to tell my arunio when to turn on and off digital pin 5. nickynichols

Yes you can.
I have not programmed the PC side, only sent info from keboard thru the USB.
Arduino side is very straightforward to receive serial input and act on it.

basics are:
check if data came in with
if (Serial.available() >0){

then read it into a variable to act in
incomingbyte = Serial.read();

then act on it
if (incomingbyte == value_you_want){
//act on it
}
}