Remove the video card, sound card, memory, power supply, everything that may impede access to the heatsink. Now grab some needle-nose pliers and turn the tab counter-clockwise. Don't try to break it as you'll probably just damage the mobo, just turn it. Once you get 2 of them loose the last 2 should be easier.
If you still can't turn it get someone stronger and less fearful to help you.