Also, you can try what That1Guy99 did to fix it by installing another copy of Arduino IDE (say, 1.0) and reinstalling the core files and modifying the boards.txt with everything in the beginning of this Topic.
@strykeroz - I found the "burn bootloader" isn't necessary with core13 for setting the clockspeed because the high and low fuses come from the boards.txt. I just select the 128kHz board and upload the sketch (Don't even have to select Upload via Programmer).