Is it true anything you post on the internet can and will be used against you in a court of law?

[Justin]

It's not that easy, all the evidence they acquire your defense attorney will have access to them as well. If they pick and choose information they attain to form a story and depict you in a certain light, your defense attorney will be able to show the rest of the information clearing up the manipulation of evidence.

The information they attain also has to pertain to the charges. If you get charged with murder but they find child pornography in your computer they can't use that evidence (judge will throw it out if they do) because that has nothing to do with the murder charges (though you probably will be charged with child pornography afterwards)