Cannot Upload anymore

Hi, I used to be able to upload no problem to my Arduino Duemilanove but now I can't upload anything. I get an error saying:
avrdude: stk500_getsync(): not in sync: resp=0x00
avrdude: stk500_disable(): protocol error, expect=0x14, resp=0x51

I had no problems before and I haven't changed any of the settings or done any updates since. My original program was for a servo that moved 170 degrees back and forth after a given time. I needed to update this however to only move when a hall effect sensor was triggered. After I installed the new program I tried running it and it wasn't working properly so I tried to go back to the original program but it wouldn't let me and gave me the error above. Again nothing was changed and there was only a 2 day time period between installing the new program and trying to go back to the old. Now I cannot get any program to upload at all. I have tried everything recommended in the troubleshooting and nothing is working.

So again, I didn't change my Comm port I didn't change which board was selected, I am running this on Windows XP and I am using an Arduino Duemilanove w/ ATmega 328.

Please help me.

Unplug the USB cable (either end).
Hold down the Arduino reset button.
Plug in the USB cable.
Click on the sketch Upload button.
When you see the "Binary sketch size:" message, release the reset button.

I've tried this as well and it does not work.

You could also try to connect the USB onto another USB port.

Do the TX and RX LEDs twinkle when you try to upload?

Do you have anything connected to the data pins, particularly 0 or 1?

I have nothing plugged into it at all and the TX and RX blink for about a second when I first plugged in the USB then they stop.

So is there anything else I can try?

Try different selections from the "Boards" menu. Perhaps you have a bootloader that uses a different baud rate. If you get success or an error saying the chip signature is wrong you have found the right baud rate. Check "boards.txt" for that board type to see what baud rate it uses for upload and then make sure the "Arduino Duemilanove w/ ATmega 328" entry has that baud rate.

If all else fails, burn a fresh bootloader.

I cant even burn a bootloader. I tried doing it with every possible option.

getandistr
If by chance you have plugged the board into one of the other USB ports from when you first put it in, it may assign a new Comm port number.

The fact the rx/tx lights flash when first plugged in, tends to suggest it is communicating with the computer.

You haven't tod us that you checked the Serial Ports that are available.
Open up device Manager (under ControlPanel System HardwareTab Device Manager look under Ports) You should see one that says Duemilanove.

I spent some time with another user who was trying to upload to Comm4 because thats what it used to be, when the UNO (in his case) was on Comm3.

Mark

getandistr:
I cant even burn a bootloader. I tried doing it with every possible option.

What ISP device are you using for burning a bootloader?

I bought a new board thinking that was the issue (this one is a UNO) but again I have the same problems. Also Serial Port Comm1 is now the only one that is showing up. I don't know what else to do. I even tried changing which USB I plug it into.