If you got the ME install disk or restore disk with that computer, the best solution is probably to wipe the hard drive clean and start from scratch. Odds are that if the seller was doing searches for porn, he may also have saved some images and/or videos on his PC. Wiping the hard drive is the best way to get rid of all that preceded your ownership of this computer.
If that's not an option and you're talking about Internet Explorer, clearing the History and Temporary Internet Files are a good place to start. Tools\Internet Options.
I recently bought a used computer for my daughter which has ME installed. The previous owner's search information is still on the system. If she is looking for "School," the word "Sex" and other words are listed. How can I iliminate this? Thanks in advance!