Hi my name is ***** ***** I will be helping you.
You can only be logged into windows one user account at a time.. You may have a trojan which can let other people gain control of your computer.Please follow this link, and select run ESET online scanner. This will find any nasty bits you may have.http://www.eset.com/us/online-scanner/
You can also see how many user accounts your computer has by going to your control panel and clicking on user accounts.
How can you be certain? Please run the above scan and post the results in here please.. Thank you.