I am sure this has been asked before but I could not find it so here goes. Logging on to several sites I go to I have found usernames that I did not put there. Is someone trying to log on my accounts and if so how do I stop them?  I have about 15 usernames I did not put there.  

I do not understand.  What sites are you going to?  The title says the usernames are on your pc but the text of your post says they are on sites.  I am assuming you mean websites, like discussion boards or forums.  Couldn't it just be someone else who uses the same user names as you do?  For example, another mom that lives in a town called morgan town.  What makes you think that they are user names that belong to you?