Ahh..I see. You may have a registry side edited by the virus. First thing to do is to backup your registry. You can either do it in registry window.
a) Go to start>run>regedit
b. Under file tab, pick export.
c) Select all under export range after giving a name to your registry file.
Now,I predict that some registry file need to be edited. It's a dangerous process but, if you have backup your registry so you can restore it if something unwanted happens. Now under My Computer, there's a lot of tree files. Jump to :
a) HKEY_Current_User > Software >Microsoft >Windows > CurrentVersion>Policies > Explorer
Find a key named NoTaskManager, delete it. It may appears on Policies tree, so delete it also.
HKEY_Local_Machine > Software >Microsoft >Windows > CurrentVersion>Policies > Explorer
The same key. Just delete it. It shouldnt be there as a default registry value was.
c) Refresh the windows for a while, or even better do a restart. Press ctrl+alt+del, and that should fix your task manager restriction via virus problem.
Ok, I think that should be the solution. You got the registry back-up so it should be good. Good luck. And if does went from good to a better state, do a system restore checkpoint.