arduino diecimila doesn't autoreset

My arduino diecimila doesn't autoreset so i have to press the reset button. when i try to upload my sketch it gives the error: avrdude: stk500_getsync(): not in sync: resp=0x00 avrdude: stk500_disable(): protocol error, expect=0x14, resp=0x51 what should i do?

How can you tell it doesn't autoreset?

Have you read the troubleshooting guide.

Because i uploaded blink sketch from the examples. the first tiem everiting ok but then the led keeps blinking and it says that error. but if i press the reset button it uploads fine. i think i doesn't autoreset. what do you think? can you help me?

In all likelihood this is your problem is you are using Windows. Set your serial port with the following settings:

Device Manager -> Com Ports -> USB Serial Port -> Port Settings -> Advanced button -> Set RTS On Close

I had the same problem and setting this solved it for me.