There are some viruses that restrict you from using the run dialog box, hide files, task manager, registry editor, folder options, and more. Usually, before an antivirus can detect it already have done its damage.
If your antivirus had cleaned your computer and still you are lockout from the task manager and the likes then you could use RRT. RRT stands for Restriction removal tool, this is a free software that you can download to remove restrictions / enable you to access the said programs that the virus have changed.