Sure you can, most systems have some sort of drainage point, usually a drain valve on the rad, and once you've drained the system (i.e. leaving the pump running while all the water goes out into a basin) then you can take apart the components relatively easily without extreme spillage. It doesn't really matter that much if you get a little bit of water on your computer anyway, as long as it's off and you dry it thoroughly before you turn it back on again
