Problem Uploading a New Program (in Duemilanove)

Hi, I have a Arduino Duemilanove with ATMEGA328. I'm experiencing this problem:

avrdude: stk500_getsync(): not in sync: resp=0x00 avrdude: stk500_disable(): protocol error, expect=0x14, resp=0xe0

I've checked the right board and the right COM port. It was working before but suddenly this error started to happen.

The firmware I coded before is still running in the board, but I can't change it!

Please, anyone know how to solve this problem?

Thanks.