Set how long you need to avoid distractions and then click the Go Cold Turkey! button. Until the timer is up, those sites will be completely inaccessible from any browser.
Even restarting the computer or uninstalling the program won't unblock them! In fact, too many attempts to disable Cold Turkey will block those sites for a week.
The free version of Cold Turkey can block websites from 10 minutes to a week. There is a paid version of Cold Turkey that extends the block time to 30 days and blocks programs, too.
If you're using this program for the first time, I recommend taking it slow. Try blocking in increments of 30 minutes or a few hours.
Also, be sure to double-check the timer before you start blocking. You don't want to accidentally block your favorite sites for a few days.
System: Windows XP, Vista, 7