open regedit first
follow the sequence

HKEY_CURRENT_USER -> SOFTWARE -> MICROSOFT -> WINDOWS -> CURRENTVERSION -> POLICIES -> EXPLORER

then right click on explorer and scroll to New -> Dword value

then new Dword value registry will be created.

Then rename that registry as noclose

then double click on noclose then another window will pop up then set the valuedata to 1

say OK

then restart Windows .....

It's done now you cant shutdown

make that valuedata to 0 to enable the shutdown.