Did I do a terrible thing?

If you haven't clicked any sites that looked illegal, then there is no need to clean up the PC. As long as you have not done anything wrong you don't need to fear. If you will do something like cleaning up the PC out of fear, you will appear guilty without doing anything wrong!
Not only that. Even after cleaning up the PC you will be anxious whether you have cleaned up 'properly' and whether the very act of cleaning up isn't an evidence of guilt. Your anxiety will increase.
So my advice is "DO NOTHING". If you are ever questioned, tell the truth.