Arduino sketch uploads only once

Hi, I am using arduino uno and have installed it perfectly on windows XP. It works fine for one time i.e. when i upload a certain sketch on arduino uno(connected to com port 3) it gets uploaded for the very first time and after that when i again try to upload the sketch to the board i get error " COM port 3 already in use". Now for uploading the sketch after getting this error i have to unplug the usb cable(from pc or board) and again plug it to the pc or board and it works fine. But this process(plugging and unplugging the usb cable again and again for each sketch and even for a little change in the sketch) is quite irritating .I dont know what to do . can anyone help me on this ?

P.S : I am using a shield while uploading the sketch but i even tried to upload the sketch without the shield but still the same happens as described above

Have you got anything connected to RX or TX of the Arduino?

No nothing is connected to Rx or Tx

Hmm, try resetting instead.