These are not classified as viruses rather they are known as rootkits I had a rootkit that did that the following things in my computer
* Disabled the Task Manager * Folder options missing * Regedit disabled ( Regedit is the registry editor ) and some
Believe me even an updated norton was not able to even detect it. I even tried other antiviruses like McAfee,NOD32,BitDefender,Avast was unable to solve its puzzle
The solution given by Microsoft is the following
http://support.microsoft.com/kb/555480
Another alternate solution to this problem is the RRT (Rootkit Removal ToolKit)
I will give you the link and this is just a 50kb of download.