I think that someone at work has logged onto my user account when I'm not in work and moved some files from my desk top to my work in progress. Can this happen? They may know my user name as this is made up of each persons name but, I don't think anyone has may password. Can they still access my desktop etc?
Yes, it's actually quite easy, assuming you're on Windows and an administrator on your computer.
Now check through the logs. Look for entries that have Task Category set to Logon. You can use the filter functionality to only show those entries. An audit success means that a user logged on successfully, whereas an audit failure means a user failed to log on. You can see the dates and times that these attempts occured.