When you’re hit over the head, the instrument could be a “lead” pipe. But when it’s a verb, “lead” is the present and “led” is the past tense. The problem is that the past tense is pronounced exactly like the above-mentioned plumbing material (“plumb” comes from a word meaning “lead”), so people confuse the two. In a sentence like “She led us to the scene of the crime,” always use the three-letter spelling.