Sometimes Registry Editor may be disabled. It may be due to a virus, but may not be.
If Registry Editor is still disabled after removing the viruses, you can use this method to get it enabled.
General Policy Method:
1) From Start menu select Run and type gpedit.msc and hit enter.
General Policy window opens.
2) Get to this location:
User Configuration > Administrative Templates > System
3) In the right pane, double click open Prevent access to registry editing tools.
Select Disabled and press OK.