See i unplugged the arduino cable and again plugged it in and checked which port it uses and then selected that com port no. in the arduino software and it worked.
I dont know that if the problem is in your arduino or not
was there any other device attached to your pc alongwith arduino e.g ipod,etc
Because when i got this error i had my ipod attached to the pc and when i removed the ipod and again tried it worked.
I really dont know that it was because of that ipod but you can try this.