Not necessarily someone's sneaking in your computer. Usually this problem can be fixed by rebooting. This happens to me all the time. But you can also fix this without rebooting.

Make sure that you are using Admin acct. Then you can stop this in the Task Manager (you can use Help of Task Manager). Then run disk check and if you don't have it down load registry repair software. Run both.

Or use this one

http://ccollomb.free.fr/unlocker/

Hope this helps.