Make sure you have the correct board type selected. When I first installed 0021 Duemilanove was the default, and I had to set it it to Uno.
You can do this from Tools->Board->Uno.
I got this same error message when it was wrong.
avrdude: stk500_disable(): protocol error, expect=0x14, resp=0x51
Should add I'm using Windows 7 64bit professional.