I don't think it is wrong of you to be concerned, especially if they are a bit whippy and decent with computers. How long ago did you loan them the computer? Is it possible for you to do a system restore back to a point before you let them borrow the notebook? Do a
spyware scan of your computer with a couple programs before doing anything else. I use spybot, malwarebytes, and superantispyware from that page. After that is done, if you can not do a system restore, can you create a new account for logging into your Windows? Change all of your passwords and account reset information (alternate email addresses, security questions & answers, etc). What exactly are you seeing on your screen? Can you
take a screen shot and post it?