I have reason to believe that someone else may be on my PC. I am the only one to use this PC and no one else is authorized to use it. Is there a way I can find out. I am worried since I had a person in to help me hookup something and they put their flash drive in the unit and I feel they may have compromised my PC.
Please let me know if there is a way to check if someone else may have access.
Thanks.

Try checking the history on the internet by pressing ctrl and h at the same time. Do it several times throughout the day or when you think someone has been using it.

Password protect it.