avrdude: stk500_getsync(): not in sync: resp=0x00

I have an ARDUINO Diecimila with ATmega 168 chip in it. I installed the drivers for the ARDUINO and after successfully installing the drivers i tried many example sketches which are available in the arduino IDE such as BLINK, FADE, BLINK WITHOUT DELAY, the serial port and the board were also selected properly in the ARDUINO IDE and the program worked superbly.

But i don't know what happened suddenly, when i tried to upload another sketch into the board the error message "avrdude: stk500_getsync(): not in sync: resp=0x00" appeared in the IDE. from then onwards whichever program i upload i'm getting the same error even in another computer

Then i referred to the troubleshooting section and tried everything possible but the problem wasn't solved. Now i'm totally confused in what to do and the final dates for the submission of the project is also nearing... someone please help me out...

i have uploaded the screenshot of the error message which is being displayed on the IDE

Is COM3 the correct serial port?

yes i have verified it more than twice

Does the board pass or fail a loop-back test...
http://arduino.cc/forum/index.php/topic,73748.0.html