It's likely you still have a virus on your system. The executable might still be signed but, it's likely the signature reports as being invalid.
A correct version of it is downloaded every time you run any of the installers but, if the virus is still active, it will modify it as soon as you executing it.
I'm sorry, but there's no other way to fix this, than being sure you remove your virus. A system reinstall is probably the safest choice.