If you pulled it out of a lake, I would bake for at least half an hour at about 500 degrees. Of course, keep an eye on it to make sure it doesn't char or catch fire. [Disclaimer: I accept no liability for any damage that may result from you following my suggestion. This includes damage to the driftwood, your home and property inside, and any bodily damage that may arise to any individual.]
Follow it with boiling to get rid of any chemicals. You might want to soak for a while (weeks to months) after that if you think there are pesticides or herbicides used in the lake. The baking is more effective at killing microorganisms because the water will only go to about 212 degrees (slightly higher because of particulates in the water). This should be enough to take out most of the microorganisms, but I always go with better safe than sorry.
Here's a piece that I picked up: