Disclaimer:  This is only for Windows XP.  It worked for me and may not work for you.  I’m a complete novice, so use at your own risk.  That said, this procedure completely fixed the “avrdude: stk500_disable(): protocol error, expect=0x14, resp=0x51” errors I was getting in the Arduino environment.

1. Download and install the Arduino IDE
2. Download the Sanguino patch
3. Apply the Sanguino patch to the Ardunio IDE (Directions copied from this page)
   1. Copy “\sanguino-software-1.x\cores\sanguino” to “\arduino-0017\hardware\cores\sanguino“
   2. Copy “\sanguino-software-1.x\bootloaders\atmega644p” to “\arduino-0017\hardware\bootloaders\atmega644p“
   3. Copy all the folders in “\sanguino-software-1.x\libraries\” to “\arduino-0017\hardware\libraries\” overwriting the existing libraries.
   4. Open the Arduino file located at “\arduino-0012\hardware\boards.txt” and append the text from “\sanguino-software-1.x\boards.txt“
4. Edit the “\reprap-gen3-firmware-1.6\ArduinoSlaveExtruder\ThermistorTable.cpp” file so that it reflects the new table information for the 1mm thermistor. (New table data courtesy of Jet)
5. Copy the entire contents of “\reprap-gen3-firmware-1.6\libraries\” into “\arduino-0017\hardware\libraries\”
6. Run the Arduino IDE
7. Tools -> Board -> “Arduino Diecimila, Duemilanove, or Nano w/ ATmega168“
8. Open “\reprap-gen3-firmware-1.6\ArduinoSlaveExtruder\ArduinoSlaveExtruder.pde“
9. Sketch -> Verify/ Compile
10. Wait for “Done compiling.“
11. Hold down the reset button on your extruder board.
12. Click “Upload“
13. Wait for 3 seconds after you’ve clicked “Upload” and then let go of the reset button.

Congratulations!

A huge thank you to Jet and Rick Pollack for walking me through this procedure step by step.