Make a new DWORD value (under HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Authentication\LogonUI\Background) named OEMBackground and set it to 1.
Got it thanks. Edit: Strange enough, I already had all those keys. I was looking under current user and not local machine.

Now I need to turn off the shine on taskbar. Any idea on how to do that? Easily found how to do it on Google a long time ago, but I can't find it now for some reason.