I'm also finished with Arduino (Uno).
I have the same (famous) problem:
avrdude: stk500_disable(): protocol error, expect=0x14, resp=0x51.
For several hours I've scanned forums for this error and followed the suggestions for solving this problem, but no avail.
I have also programmed other micro controllers (from motorola 6800 to msp430-xxxxx). There was (of course) som minor problems, but I was able to solve it in short time.
I've tried it on two computers; my XP laptop and a Server 2003 (only SQL server running), but the same error.
The funny thing is that there when I'm using the "Serial Monitor" and TX- RX pin connected together, I get feedback...
I was lucky once, I managed to upload a modified version of "Blink". Thats all...
My Uno is now in the dustbin!